I wouldn't even bother converting them. First off, Simms loves for people to think that the felt soles are the main way hitchhikers are spread when in actuality it is the insoles, tongue gussets on the boot, and gravel guards that hold the majority of hitchhikers. Tons of articles by independent studies that show this. Sure, felt will hold hitchhikers, but switching to felt does not mean you shouldn't clean your gear before going from one watershed to another. You need to clean all of your wading gear to keep hitchhikers from spreading. You really don't get an advantage from Vibram soles other than easier walking on the bank and on gravel. Both felt and vibram boots have to be cleaned the same way, so I say if your felt is still good, just keep it until they wear out, then visit the Vibram alternative if you want.