You can use 1.0 saves in 1.2. At least my saves of an England campaign in 1.0 loaded and I played couple of turns, and everything looks fine.

It might be connected to the thing that 1.2 patch is from the foundation patch. It bases it's changes on 1.0 not 1.1. So some of the changes of 1.1 may have not beend done the same so they are not compatible (people reported crashes when instaling 1.2 over 1.1).