Not sure if this is covered elsewhere in the forum, but I have discovered a useful workaround for working on files that have been imported with a 'previous version' preset.
The problem arises that as soon as any of these files are opened in Develop module and any of the 'update process version' options are chosen, LR4b hard crashes.
Temporary solution unitl the bug is repaired: Don't go to the Develop module! Select all the relevent images in Library module and go to Menu > Photo > Develop Settings > Update to current process (2012).
It works for me 100% of the time on my MBP i7 with OSX10.7.2
YMMV so add to this discussion if you tried with a good or bad result.