OK, I tried installing 1.2, upgrading to 1.3 with the 1.3 patch, and then attempting to load my known good 1.2 saves. It seems that, as others have mentioned in other threads, that a patched 1.3 RTW install will, sadly, not load the 1.2 saves. It is astounding that this is true, but it evidently is. So the only real work-around that I can currently see is to maintain 2 separate installs... a 1.2 version and a 1.3 version.
As a further comment, I have not played a lot of the 1.3 patch yet. If what I have read in this forum is true, particularly about the horrible bugs in Roman faction play introduced in 1.3, then it looks like I'll need to keep a dual install anyway, depending on exactly what I want to play (e.g., which set of these terrible game bugs I would enjoy infesting my game). I'm really really hoping "they" read these sorts of posts and immediately and firmly commit to a 1.4 patch which is thoughtfully considered, well-programmed, and properly tested... which will elimiate these sorts of gameplay (particularly AI) bugs.
So, it seems WE will be unable to play 1.2 files in 1.3 patched games, and I do not have the time to make a conversion program (but... maybe someone does!) for 1.2 saves to become 1.3 compatible.
Good hunting....