In 2009, the 1X6X1 arch had its curve raised and the new top point of the curve now sits one plate below the top of the brick. If employed as an arched top to a door or window the difference is negligible but for other uses, such as adding inverted slopes underneath, it introduced an unwelcome vertical facet.
There was a clear benefit to the change too; a 'telephone bow' (part 93273) now fits inside the arch, and of course all bows with the same shaped curve do as well. This serves to highlight the important fact that the height to which the arch has been raised is less than the height of plate; it's the same as the height of the 'lip' on bows and slopes. So you can't put a full-width plate inside the curve of an arch.
In 2011 it was the turn of the 1X6X2; the original version being amongst the most beautiful of LEGO bricks in my humble opinion thanks to the semicircular curve and its proportion compared to the brick itself.
Dark Age and learnt of SNOT, one of the first things I tried to make was a perfectly round window from two of these arches. I quickly realised that this was not possible - the semicircle is in fact not quite complete. It's worth mentioning The Hobbit 79003 An Unexpected Gathering contains a near-round window created with two of these old-style arches, using clip light plates to set them at 180° and separate them slightly to fit a square 4X4 window pane (made of turntable bases) inside. It's really beautiful and a very advanced technique for a kids' set - actually, 79003 is one of my favourite sets of recent years.
For the new 1X6X2, the near-semicircle has been raised (again, by the height of a lip) which has resulted in the 'round window' technique looking much better, as seen above - but it's still not a perfect circle. The important thing is that you can now insert a 4X4 rounded part in the hole, with some small gaps, without needing to separate the two arches. Sweet.
Notably, the top of the 1X6X2's curve is fractionally higher than on the 1X6X1. I am guessing this exception is because TLG wanted to be consistent about raising all arches by the height of a lip and did not wish to change the shape of this particular curve - i.e. it was necessary for it to remain perfectly round. This additional height, plus the removal of the reinforcing bar inside the arch, permits a technique with the new 1X6X2 that none of the other new arches do: a centred plate can be placed in a stack of five underneath, as you see below. I doubt that this is the answer as to why this arch is different to the others, as this technique isn't terribly useful. I'd guess it's just an accident of the geometry and might even be considered illegal by TLG, if those parts are in fact pushing against one another at a microscopic level. But it seems fine to me.
Next came the new 1X12X3 arch, which BrickLink lists as first appearing in 2014 sets, but I got some in my Friends 41015 Dolphin Cruiser last summer. Which is another fabulous set actually! Doth arches maketh the set?
As with the others, it has been raised by the height of a lip. Again, no existing bows fit the new curve perfectly but I noted you can place a 45° slope underneath quite snugly - however there's no room to add anything onto the studs on top of the slope. Unlike other raised arches, the topmost point of the curve has remained in the same place as the previous version, which in this case is two plates from the top of the brick.
So as I began by saying, what we are now beginning to see in sets is the raised 1X8X2 arch; again with the bottom of the curve raised by a lip and the top one plate below the brick's top. Like the 1X6X2, the reinforcing bar underneath has gone.
There's no guarantee you'll get the new version in the following sets whilst TLG use up the old stock, but for the record it might appear in: 70133 Spinlyn's Cavern, 10674 Pony Farm and 41057 Heartlake Horse Show. BrickLink are yet to add this variant to their catalogue but if you're simply dying to get hold of one, BrickOwl have listed it.
Another aspect to the new 1X8X2 as well as the 1X12X3 is bound to irritate many AFOLs. The interior walls have been thinned, meaning you can no longer attach parts along the curve (as shown here with the old-style arches).
I've heard many, many AFOLs claim the thinning of walls (and other general changes like the tiny hole in the tiny pins on the undersides of bricks) is a cost-saving measure by TLG... i.e. using fractionally less plastic in each brick. Maybe it's a factor but I've never believed it is the primary driver; in this particular case I feel it's far more likely they're deliberately removing the ability to make connections that are weak and/or not in System. Ironically enough, the technique was actually used in an official set in 2007 - the seminal 10182 Café Corner. Perhaps it was later felt that the single point of connection was pushing the limit of TLG's internal guidelines of model stability. Or more fundamentally, the parts were deemed to be being used in a way that was out of System.
It's worth noting that on the smaller new arches, the walls haven't been thinned (or not that I can tell; parts still kinda stick to them). Maybe this is because smaller parts need more strength. Or maybe the Parts Designers are really really mean and know that AFOLs don't tend to use this technique with the smaller arches!
There is another arch I've not mentioned whose history is a little more complex, as it received more than one redesign - the 1X5X4 half-arch. First it got thinner walls, but then a rather odd change was introduced.
The curve was not only raised but also the top point was brought one stud inwards. This enabled a connection point underneath the topmost section of the arch - not a massively useful change. A couple of major benefits were lost too. The curve of the old 1X5X4 described a quadrant and so could be used to make a perfect 8X8 round hole. Also, the 1X6X3 1/3 half-arch (part 6060, seen in White above) used to fit perfectly underneath, whereas the new design leaves gaps.
[EDIT: Andrew Barnick and other readers have pointed out important news; the 1X5X4 arch has now reverted back to the same original curve as 2339! Rumour has it that TLG considered the curve of 76768 to be a mistake - so I guess this news confirms the rumour. The new part number is 14395. The walls are, of course, thin.)
As with all design alterations, there are pros and cons. TLG obviously weigh these up, and involve different departments in the decision-making process. Of course, the pros and cons as far as AFOLs are concerned are somewhat different, since TLG are driven by what's best for kids and keeping everything within System. In the case of these arch changes, I'm still a little puzzled. Changing the top of these curves to be one plate (or two) below the brick's top feels nicely in System, but as mentioned the 1X6X2 breaks the rule. As to the benefit of raising the curves by the height of a lip - perhaps one of you can enlighten me. Only the 1X6X1 shows any clear advantage to me... in fact, taking the lead from the telephone bow combination, I'd love it if the reason is that TLG intend to release new bows in the future designed to fit perfectly inside other arches. A semi-circular smooth brick to fit the 1X6X2 would be a really nice decorative part!
So, is this the end of the arch-raisings? The top of the curves on the 1X3X1 and 1X4X1 are already one plate from the top of the brick, so we're all good there. The 1X3X1 already has generous verticals at the bottom, larger than a lip. The 1X4X1, being based on a perfect circle like the 1X6X2, should not have the shape of the curve altered otherwise 2X2 round parts will not fit inside. I doubt it could be raised by a lip in any event, otherwise surely the top of the brick would become too narrow?
Love or hate the changes to our beloved arches, the fact of the matter is we're lucky enough to be able to pick and choose which style we want by buying from parts reseller sites... within the constraints of budget and colour availability of course!