This solution is possible but very difficult for even an experienced technician. What you would need to actually replace is the Baseband Processor 337S3833.
I have a place for you to purchase them.
Buy Baseband Processor iPhone 4.
Then the removal and replacement is where the fun starts and stopped for me, instead I found the next alternative easier.
This alternative to this would be to buy replacement Logic Boards that are already unlocked. And then to sell yours maybe on eBay, or through a Forum.
I actually have several iPhone 4 Logic Boards for sale either in 16Gb or 32GB all unlocked. But I have not finished building my new Website where they will be available for anyone to purchase. The iPhone 4S Logic Boards will be available for sale from mid April in all memory configurations. Also iPad 1 Logic Boards also available in both 3G and non 3G.
These boards are all Brand New and Unlocked for Worldwide use on any provider.
If you would like to purchase any of these boards from me now, then mention so in a reply to this, and maybe we can exchange contact information with Admin approval. Do not post personal details here.
I hope this helps you in some way.