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Dodging Verizon's New $30 Upgrade Fee - MarketWatch
Port the number to Google, then forward it to your new phone. Don't bother porting it back to Verizon. (Why keep switching it? The Google forwarding works just fine - and if you decide to go to another carrier, there's no porting to get messed up, you just forward to your new number. (And you can forward to more than 1 number. My local Google number forwards to both my cellphone and my home phone, so they both ring. Whichever one gets answered gets the call.)
(I hope AT&T allows the same trick if I decide to upgrade some time in the future.
BTW, your phone is listed for from $78 to $165 in fair to good condition on
Swappa, so even at the low end, you'd get more than twice what Verizon is offering for trade-ins. (The one listed for $150 is a little dinged on the corners.)