Is Apple able to change an iPhone 3G Baseband 6.15 back to its' original baseband?

Unfortuately you cannot. Your phone is now completely unusable.

By jailbreaking the phone and changing the baseband, you (or whomever) have done many things:

1) voided the phone's warranty

2) eliminated the possibility of getting help from Apple, including this forum (where discussion of jailbroken phoes is against the Terms of Use of this forum)

3) most likely rendered the phone permanently unusuable.

If I were you, I'd give the phone back and buy one from an authorized dealer.

hey,, i am having the same prob as @avilesj ,,unfortunately i was not aware that my phone was also jailbroken now its baseband has got corrupted...now since the phone's warranty has exploited is there any way that i can replace out of warranty phone and specially the one i m having....look the phone is of no use now i know that but still any way under the terms and condition of apple that can b approached to get out of this prob,

Nothing has changed in the year since this posting was created.

Either return the device for a refund to whomever you purchased it from or accept that you threw away money and buy a new device from an authorized seller.