I've used a lot of rename utilities (MultiRen is my fave, followed by BulkRename), but for Poser libraries, I put the "ren *.pz2 *.cm2" command into a batchfile called PZ2CM.bat (along with a reverse version called CM2PZ.bat), and copied it into each folder I want to convert. Dbl-click on either *.bat file executes the command without calling up a dos window in each folder. Works best for me.