Idea: don't change Act 6
I know, you probably saw the title and came here to tell me off for even hinting at not changing Act 6, but hear me out...
What if an alternate version of Act 6 with the changes discussed were released, but the harder Act 6 was also left in as a challenge for those who want to try it?
The easier Act 6 would still offer the same rewards and allow progress to the next story components when released, but would also provide those who have completed Act 6 a chance to go back and experience the new version while still being rewarded.
This would alleviate the problems surrounding compensation for those who have already done Act 6, and allow summoners who feel stuck a chance to progress. I don't think allowing double the rewards for those going through both easy and hard versions would be particularly game-breaking, especially for the amount of work it entails. This also would provide some new(ish) content for people to work on, and help those who don't know if they should still work on Act 6 or wait for the change.
The primary issue I see with this would be those who have completed a portion and not want to repeat that content even on a new, easier version. But, the prospect of double the rewards I'm sure would help put some at ease. As it is, it will be very difficult, if not impossible, to satisfy and accurately compensate everyone at different progression levels in Act 6 anyways, so why not just open both easy and hard versions for everyone to play?