Thanks for all the suggestions!  I've tried Poser's morphing tool, but the dress doesn't have enough vertices to make it smooth enough.  I tried a quick displacement map using UVMapper and Photoshop (per PhilC's suggestion) and that will probably be the way to go, but I have a long way to work on it to get it to line up.  

Since I only have Poser and Photoshop, is there a way I can more easily line up the panties on the dress?  (Of course their UV maps don't line up, so I've been trial and error... lots of error!... so far.)