I went ahead and bought the phone at Target. The 1st page of the manual had a phone # to call "if you want to keep your old number".
I spent 40 minutes with 4 different people at Verizon, none of whom could figure out how to port the old number to the new phone. Only one person there even understood what I wanted to accomplish, but she couldn't get whatever software she was using to allow the transfer to take place.
So I'll return the phone, and try with ATT...maybe they will know what they're doing.