I checked one replay and it seemed to run just fine. Set your launch options to -allowIncompatibleReplays, if you haven't already. None of the changes affected any unites, so the replay save and load functions should be unchanged. All replays should play fine.
I know this parameter and i made use of it since a very long time. Same goes for the change a replays version function that CELO offers. The problem is thats not enough.
Sometimes it works, sometimes it doesn't sometimes it seems to work but in fact you see a different game because the parser isn´t able to read the data or the AI can´t handel it, i don´t know the reasons. Besides there is this big wonderful synchronisation error message in the middle of the screen that you can´t hide. CoH1 had an console command for this, CoH2 hasn´t.
Can't you still watch them as they didn't affect gameplay ?
I guess the reasons are all the chances that Relic makes. In the Patch log you read something like bug fix but you will never know what this means and whether it does affect replays or not. In the past i casted games of old version replays and it might seem that you watch the game the players played but in fact i saw something different, the AI did things that made no sense and that were obviously not done in the original game.