This probably isn't what you're looking for, but the concept is nearly alike:
http://fb.countd.eu/cgi-bin/fbentire.pl?page=111210231659The_Guest&path;=nospamstory
It's the Role Experiment story, where a scientist swaps around the minds/personalities of different people. It starts off with Mikey getting switched with Jon. Eventually, Jon's mom switches with Zoe.
I did search, but I couldn't find exactly what you were looking for.
