v3 default had flat feet. V4 default has bent down feet. This may cause some trouble

your best bet as markschum mentioned, is export the L and R foot as objects and then reimport

your original idea will cause massive amounts of headaches, it will break morphs, morph injections, maps and on and on.  It can be done, but in general, you don't want to.

You can also try the various automatic clothing conversion programs such as WardrobeWizard and Crossdresser

Lyrra