Iphone unlock HELP!

[eblend]

Hey guys,

I know some of you have good knowledge of this stuff, so I was wondering who had any experience with this config.

I currently have 4.2.1 software, baseband is 5.15.04 and bootloader is 5.09

I want to unlock my phone to use with softbank in Japan. I understand that there is no unlock for the 5.15.04 baseband, and my only real option is to upgrade to 6.15 baseband, at the expense of GPS functionality.

Is it possible to downgrade from 6.15 to a lower version baseband with the 5.09 bootloader? I know that with 5.08 bootloader it is possible with fuzzyband, but I have 5.09 so it is out of the question. The internet has so many threads on this type of subject, but can't find any definate answers.

Anyone have a clue?

[cosmok]

3G?
http://www.beijingiphonerepair.com/fix/how-to-downgrade-iphone-3g-baseband-5-15-04-with-fuzzyband-4-1/
http://www.iphonejailbreak.me/downgrade-baseband-5-15-04-to-5-13-04-then-unlock-with-ultrasn0w-3g-only/

[tha_bandit]

upgrading to 6.15 is the easiest, you wont necessarily lose GPS, only happens on some phones, dont exactly know why, but I have yet to experience it.

You wont be able to downgrade and also any restore/updates in itunes wont be successful without the help of redsn0w, etc.