Both directions. The .craft files themselves can be physically transferred to the new install's VAB or SPH folder so you can load in any old spacecraft you've designed, but since ASAS, SAS, and Pod mechanics are different, you might have to dump those parts and re-attach the new versions to get them to work correctly, or in some cases tweak this aspect to get fully functional spacecraft.
Essentially, some pods and control items now have stabilization wheels with a certain amount of torque (listed in description), so you might have to add more to get enough control. They also use electricity when turning the ship, so make sure you have enough power being stored or generated to do this. From the forums, it seems that some old designs don't have enough torque to hold position. On the other hand, you can now leave SAS on and still perform maneuvers, so you'd be able to turn with it on, and when you let go of the control, it will then lock on pretty much where you stopped your input.
In short, some craft will be more stable because of these changes, and some less (or even unflyable) until you overhaul the control system, so be aware when using your old designs. I'm only just now playing with it, so haven't run into any particular issues I've heard described, however.
While I've been waiting, I've been designing some of the ships and station parts I want to use for the Let's Play I'm planning, so now I should be able to start recording the play sessions. The idea calls for a lot of post-production, however, so it will be a while before episodes start getting uploaded, but I may use you guys as guinea kerbals for some test footage.
