[Q] Question about ordering from TMobile and activation process

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http://forum.xda-developers.com – So I want to take advantage of the HTC One $100 trade-up promotion which has a purchase deadline of 5/19. I ordered the unlocked version from Letstalk.com and they pushed it back until the end of May. I'm thinking of ordering it from T-Mobile's website instead. My question is in regards to the activation process. My current AT&T's billing cycle ends on May 27th. Thus, I don't want to switch service until then, but I must meet the 5/19 HTC One Trade-up promotion deadline of 5/19. Can I go ahead and order from T-Mobile, get the device, and not activate it until 5/27? The number porting do (HowTos)