Absolutly no problem!
You Drag & Drop directly your files stp, sat from MOi inside 123D.
(Drag & Drop is just more easy but you can use Menus also)
Modify them.
Export as STP & SAT for MOi from 123D.
Save as .123dx from 123D if you want keep the scene in the owner format.
Exit .
it reload the .123dx saved by Drag & Drop...
Absolutly no problem!