My Family Tree "F" number was changed to a "T" number by GEDmatch several weeks ago. I've had no problems with the change. I belong to the Cajun Cousins Genealogy & DNA Facebook group. Most Family Tree testers manually changed their numbers in the group's files, but when I've entered numbers for matches on GEDmatch, "F" numbers of members who still have not entered new "T" numbers continue to function for matching. That could change any time, I suppose.