Compatibility between different Quartus versions

The /db directory, which is the database of the last compile, definitely won't work, but when opening it in Q5.0, it should give a message saying it won't be able to open the db and ask if you want to get rid of it, which you can say yes to(or just delete the /db yourself). The next issue is there will be assignments in the .qsf that Q5.0 won't recognize, as they're assignments added since that release. It should error out on those when opening, so just delete them in the .qsf and try again(I would be surprised if it's more than 1 or 2). I believe that should do it. The only other thing I can think of is if the actual .qpf format has changed. You could create a project in Q5.0 and compare.