there was a case where newer incompletes couldn't be resumed if a user went back to 4.8.1 (I think there may have been similar situations with later versions)

However, I don't recall a problem moving ahead to versions.

[edit: correction--there was a problem with 4.9.7, but it was fixed http://www.gnutellaforums.com/showth...&postid=147360 ]