From what I've heard, you can bitch and moan a lot if you upgrade from the Prevail. Depending on how long you've been a customer with Boost they might let you slip by without paying the $5 surcharge for the upgrade. This was months back though and you might not have luck trying this on this late... But it is worth a shot.
Call. Get one rep. Bitch moan. Get upgrade. If not then rinse repeat.
Just be aware going in that you're trying to appeal to the customer rep's good side. At no point in time should you start and go get nasty. Threaten to close out your account etc. When you hear about the $5 surcharge do not talk as if you've heard this before. This is the first time you've come across this notion! Preposterous! Do not try to cut to the negotiations part. Let them take their time to get to the $5 surcharge portion of the upgrade and that's when you need to pull out your What! Really! and Why! flashcards.
Be prepared to be turned down a few times before you hit the jackpot. Some reps might be new and might not even know how to do this.
If you're upgrading to the Design then expect stiffer resistance specially if you live in an area that has 4g.
Duke this out in your head a few times playing devil's advocate. Know that whatever arguments you have been able to come up with will be shot down within the first 3 minutes and learn from your subsequent attempts. Remember that Boost's customer base is made up mostly of low income, credit less clients. They expect their clients to be semi-thugs. They do not expect you to go in having gleaned knowledge from Android Forums or any other organized source. Play that to your advantage. Pretend to be the thug but with the sharp wit of the background that you hold.
Good luck.
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Android Forums - View Profile: wetbiker7. He had a grandfathered Prevail plan and upgraded to the Marquee. He kept his grandfathered plan though. He has a post about it somewhere in the Prevail section but I'm having a hard time finding it...