Not entirely, most save games fail to load and will lead to crashes to desktop, some produce a blank black screen that might be convertible to a functioning, albeit a little unstable campaign by hitting Esc to bring up the in game menu and then clicking on resume game, and some may even load without problems, but might or might not crash later on.
So, the majority of save games will not work and it's better to finish your campaign first before moving on to VI.
The replays, you can still watch by renaming their file extensions, however some odd things may occur that can lead to different outcomes than the actual one, but that problem is more prominent when viewing VI 2.00 replays with the 2.01 patch installed, in which case you would need to swap the TW.exe.