Yes it sounds like your plan will work. I would not put the "dumb" phone on the ULD plan, just wait to sell the 4s after you get everything switched.
It does look like some folks have had success switching the phones online.
Ok, good.
As far as the data plan being activated on my Mom's line, will it automatically come off once her dumbphone is reactivated on her line or do I need to speak with Verizon or change it in store?