Not sure I'm fully comprehending...
If you're only getting one phone, you can EITHER get it at Verizon proper (my suggestion) or Verizon Walmart... obviously you can't buy one phone at two places... so I'm not clear on how you'd use your upgrade at Verizon & trade in your old phone to get a $200 credit toward your new iPhone... but then take that $200 credit to Walmart & use it to buy the same phone, using the same upgrade. Lol, am I misunderstanding what you're wanting to do??
You know you ONLY get the $200 from Verizon if you actually upgrade, right?
You can't go get $200 for your old 4S, THEN take that upgrade to Walmart. The upgrade is gone... you used it to get the $200 rebate card!!!!
Btw, they take two weeks to get there in the mail- so you need all the cash up front.
In a nutshell- at VZ proper, you can pay $299 for the 6+ & get reimbursed $200, making it $99.... at Walmart you can pay $279 & have it stay $279.