I have had really good luck switching a few of mine. My ball, brb, and gtp were bought with the previous owner saying they would never take f/t. My ball and gtp took to them immediately. It was as if they had always eaten them. The brb took alittle more time.
I thaw in water with all the feeders in a bag. When switching from mice to rats thawing them in the same bag really crosses the scents and I have yet to have an issue going from mice to rats.