Sit tight. I don't know if there will be general rebuilds, but I know now is not a good time. Considering the higher end of parry characters were achieving was +8 (without two-weapon defense), limiting it to +7 is hardly a reform.
I don't have a problem with the change, and I also don't have a problem with picking +5 versus any other number. However, I do have a concern with the fact that this change fundamentally changes the math of how you build a character, and at least some players (based on the comments here) have taken a build path that burned up valuable feats which in retrospect now have no purpose or benefit. I don't mind you guys running a short term test to see how the change works (I am used to being a test guinea pig in things like this), but if this is going to end up being a permanent change, I believe the only fair thing to do at the end of the test period is to allow people to rebuild their characters, which would reflect what they would have actually done had these rules been in place at the time they first made their characters.
Alternatively, you might consider another option that would also not require any rebuilds: instead of giving an AC bonus every +5 parry levels, give the same AC bonus at +6 parry levels, and don't put a cap on the AC bonus achievable. By increasing the interval, you effectively reduce the AC bonus without killing off the utility of improved parry or skill focus (parry). To use the example of a +8 to AC (which seems to be a concern), it currently takes a skill of 40 to get an AC bonus of +8, but that same skill level at the new intervals would give only a +6 to AC.
Given my previous note that a standard, naked, unbuffed max-DEX build could get as high as skill level 37 (23 points, +7 DEX, +4 improved parry, and +3 skill focus), for a total AC bonus of +6 without any gear or anything, I think you are at least approximating your desire for an AC cap near the +5 range, without needing to have anyone change their builds. If this interval isn't big enough to alleviate your concerns about AC bonuses being too high, you could change the interval to every +7 points of parry. This would change the maxed out naked build in my example to a mere +5 AC (and you would have to somehow pump up your skill points to 56 or higher to get the worrisome +8 AC bonus you reference).