iPhone 3Gs 3.1.3 Baseband 05.12.01

[artious]

My phone is currently jailbroken and unlocked on 3.1.3 baseband 05.12.01.

I will need it to be unlocked because im on T-mobile.

Is it possible to jailbreak and unlock on 4.2.1 or any iOS4 firmware without having to update the baseband to 6.15.00?

If it is possible is there any tutorials avaiable or any information that could be useful about this?

Thanks in advanced

[i.Annie]

Do you have SHSHs saved for 4.2.1?

Do you also happen to know if your device is new or old boot?

Find if you have SHSHs with tinyumbrella. Download it at thefirmwareumbrella.blogspot.com and click save SHSHs.

Find which boot you have with idetector from ih8sn0w.com

I ask about SHSHs bc you Need those to restore to anything that isn't 4.3.

I ask about which boot bc if you don't have SHSHs, new boot to 4.3 is tethered jb and old boot is untethered.

[artious]

Do you have SHSHs saved for 4.2.1?

Do you also happen to know if your device is new or old boot?

Find if you have SHSHs with tinyumbrella. Download it at thefirmwareumbrella.blogspot.com and click save SHSHs.

Find which boot you have with idetector from ih8sn0w.com

I ask about SHSHs bc you Need those to restore to anything that isn't 4.3.

I ask about which boot bc if you don't have SHSHs, new boot to 4.3 is tethered jb and old boot is untethered.

Sorry for the late response.
According to tinyumbrella i have SHSHs for 3.1.3, 4.0, 4.0.1, 4.0.2, 4.1, 4.2.1 and 4.3

iDetector kept crashing on my system for some reason however, according to device manager, its 359.3.2 and my serial looks like this xx948xxxxxx, so I'm assuming I'm on the new bootrom.