It would take more than re-gauging the rail cars. BART uses third rail, so trolley poles would needs to be added to the BART cars since the museums do not use third rail. BART also doesn't use standard couplers, so BART's couplers would need to be replaced with standard couplers in order for the BART cars to be interchanged (switched) with the other equipment already at the museum. The last problem is that BART cars use high-platform loading, not standard step loading like the historic cars. You would either need to rebuild the BART cars so they had steps for loading, or build a platform that the BART cars could load and unload from.

Given all of the changes that would need to be made to BART's cars to make them usable at a museum, the cars would lose their historic value (if any).