There are also some dynasty bugs we've uncovered in 1.04a that will probably be embedded in a save and will (eventually) come to haunt you. And there are a ton of kinda screwy things that will happen as a result of other changes...not to mention that your own nation is likely to be kinda messed up if it was initially built under 1.04a. I would image that all hell might break loose as soon as you launched the save.
I think you'll find that playing with the beta is *very* different -- more challenging, more entertaining, and probably the single most drastic change to a Paradox game ever to result from a single patch. You'll definitely want to carefuly read the sticky of all the changes made since 1.04a since there are some very fundamental alterations to game play that will take a while to get used to.
EDIT: the master change log is
here.