I didn't change the actual IMEI of the phone. AT&T asked for my iPhone's IMEI and I gave them the one for samsung. They changed my iPhone Account's IMEI.
Ehh..doesnt matter. At&t will sweep and it will evetually be taken off. How do I know? My friend used his 3G for a very long time with an original $20 iPhone data plan - he would receive emails like every month saying the plan he was using was ineligible, he would call and complain and give the IMEI of an original iPhone - but eventually at&t would catch on. Not worth IMHO.