Unfortunately this is not a valid option. I spoke to Sprint about doing this and was told the sign up phone credit is ONLY available when signing up, and if I chose to activate my own phone I would lose it. I was hoping to do the same thing as the above user. I think it's a poor business model personally as this only prevented me from signing up for sprint at all, and has me seriously considering going with Verizon now since they are offering double data and GNex for $99. Unless Sprint releases this phone soon. By the 2nd qtr this will only be a mid level phone with current specs and a terribly dated. "Flagship" Being cdma it's already lost pure Google phone status, Sprint is really dropping the ball postponing the release date til after MWC.
End rant :-$
I'm willing to bet the rep you spoke with doesn't have 8+ years Sprint experience, nor likes what they do for a living.
If you live in the state of MA or anywhere in SWFL, i will give you the name, number, address of nearly 10 locations that can accomplish this for you, as it's normal business practice.
If you walk into any sprint store, with a clean, ready to activate ESN, and say you want to activate a new line of service, you will have reps jumping over one another to help.
They don't have to 'sell' you anything, they simply plug your ESN into the appropriate field, and away you go.
"Sign up phone credit" no such terminology.
The upgrade credits are system based, and based on activation of an ESN that has NEVER been activated before.
So unless your 'used' device, has never been activated on the sprint network previously, your upgrade credit will stand as normal.
Still skeptical?
Do the following: Proceed with activating your used device to a new line. (you have a 14 day trial period to cancel), I'd say give it maybe 24hrs, or even 48hrs, or day 13, i don't care, then ask sprint or look up online @ sprintrebates.com with your number, and see what promotion is available to you as an upgrade.
There is your answer if it says you have a $150 credit towards a new device (which in itself is a fictitious number, since the available promotions on upgrades are always better than $150 off retail pricing).
If the upgrade isn't there, then I'm apologizing upfront. But you would be the first person to prove me wrong in this situation, as described herein.
Enjoy