[Q] Switching out the motherboard in the i337

Hi everyone! I have a bit of a dilemma.

I bought two AT&T S4's from a seller from on eBay. One for my Mom, and one for me. Now, the listing says that the phones are guaranteed to have a clean ESN and IMEI, are activation ready, and all that good stuff! So I get the phones yesterday and check both IMEI numbers and they check out. I went a step further to see if I could get them unlocked, because hey, why not? No dice. One phone's service contract hasn't been completed, and the other phone's Next payment plan hasn't been paid off. So I'm sitting here like . . . . okay, so will they both eventually get blacklisted? I sent the seller an e-mail about it. I could care less about unlocking the phone, but I do care about having the IMEI blacklisted some day out of the blue and not being able to use the phone, but that's another story for another day.

If the seller gives me some push back and things don't work out, I was thinking about switching the motherboard's out. I know another i337 motherboard would work just fine, but what about an M919 motherboard. Are they interchangeable? Just wanted to know! Thanks :)