What if I would purchase the phone on a monthly payment option then after I pay it completely off, switch the phones afterwards?
Think it'll work?
No, they put the kibosh on that loophole.
S5 tap'n
Not exactly. OP said "after I pay it completely off, switch the phones". Assuming he means after the 2 years to pay it off, in that example there is no restriction to placing that phone onto the Unlimited data plan. It's no different than buying it completely outright at retail. Only difference is it would have to be part of a new phone number activation and remain on the new phone number with its own new data plan for the entire contract length, whether 1 or 2 years, and would have to be free of all financial obligation before being moved to the Unlimited plan.
Now, there are (as shown in that link above), ways "around" this and if done carefully has yielded success however we are neither confirming or denying such existence nor are we in any way guaranteeing that it will work for you, even if it has worked for others in the past as reported by them. YRMV!! Proceed at your own risk.