do not save a cs4 project to a cs 5 project it will kill your prjoect. ..
my project was 8 years old from macromedia MX to adobe flash cs 4 but . . .
saving it as an flash cs5 was impossible.
if you have ANY ANYYYY empty frames on layer one, on your project, it will kill itself by hanging.
so any work you dont have backup on is blown up.
it has cease to exist
it is no more
it has kicked the bucket.

and overall its not so bad