We use numeric then alpha. Some of our older products used a scheme of (-,A,B,C,...)
What we decided to do was no longer use '-' as a rev, but in order to switch versioning schemes the versions in the current scheme must be in the target scheme. So we added '-' to the front of the numeric scheme and set as legacy='true'. This allowed objects that already had '-' to switch versioning series, but any new item started at '1' instead of '-'
attached is our filebased series.