Look in the boinc data folder for a file called "lockfile". If it's still there when the client isn't running, delete it. I THINK that should take care of the client not being able to use that data folder.

As for changing client versions, if you go back too far they store the data in different formats and the older client can hose data from the newer one. I wouldn't have expected that to be a problem with everything v7 plus though and thought it was only a problem with v6 clients.