You might need to replace the level.dat file as well.Sorry if bumping this post is obnoxious.. I don't really want to play until I get this sorted out. I'm kind of in a pickle between having to do all of my research again or having to rebuild all that I had built between my last backup and the older backup.
From my own testing the playername.dat does nothing in SSP unless you change the level.dat entry as well.