So I upgraded to 4.5.1 and dutifully followed the instructions given to create a new workspace, except that the instructions seem to leave out a step. After doing the exports and creating a new workspace I tried to use Import/File System to transfer my project but FB complained that it couldn't import into a workspace with no open projects. Hmmmm....didn't see anything about that in those instructions. So I created a new project named the same as my old and did the import: now when I try to launch it Flash complains of a "SandBox violation" and it seems to be looking for a crossdomain file.
I didn't have this issue before, launching my project hundreds of times. I realize a million things could be wrong but I tried to follow Adobe's instructions (such as they were). Seems to have been a mistake.