Why would I need the old IMEI? After soldering the new chip on the motherboard, the new chip brings new IMEI to the phone. The phone has no warranty so basically I don't need any proofs for the IMEI or that stuff. I don't care about that at all.
Check the Desire S EMMC replacing thread on this forum and you'll see that the phone got new IMEI with the new chip and it works just fine.