I know this has been reported before, but this must be seen by Mojang. Seamless slabs have been replaced by nether brick slabs. I am aware that seamless slabs are still available by using damage value 7, but what about the millions of builds that already use these blocks? They'll be ruined. Really, it's not so much a bug as it is a broken update. If an update is reported to break existing worlds, then the update should be considered broken and an effort should be made to fix it.
My suggestion is that nether brick slabs be assigned the damage value 7, leaving seamless slabs intact, or that a new command should be added that allows the player or admin to replace all of one type of block with another, like the functionality WorldEdit provides.
From what I've seen of the upcoming update, this is the only real flaw. It's serious enough that I'm considering never updating or even changing my password to something so immemorable that I can't log in, preventing accidental updates in the near future.
Please don't resolve this as "invalid" until it gets an official response. This is probably my most likely way to inform Mojang of the situation and resolving this serious problem as just another invalid non-issue is not helping.