A lot of users confuse between downgrading firmware and downgrading baseband. You can downgrade firmware to an earlier version if you have SHSH blobs saved. But you cannot downgrade basebands by just downgrading the firmware.
As we said that normally its not possible to downgrade basebands but this is a special case. iPhone 3GS users were given an option to install iPad baseband which is 6.15.00 and unlock their devices.
iPhone 3GS with basebands 04.26.08 / 05.11.07 / 05.12.01 / 05.13.04 can be unlocked using Ultrasn0w. iPhone 3GS with basebands 05.14.02 / 05.15.04 / 05.15.04 / 05.16.01 / 05.16.02 cannot be unlocked with Ultrasn0w so they have to install iPad baseband 6.15.00 on their 3GS device which can be unlocked. That was the whole story.
Now we have seen a lot of iPhone 3GS devices using 6.15.00 baseband to get the unlock. Once they upgrade to this baseband, it cannot be downgraded back to any earlier baseband. Now the question is why would they want to downgrade 6.15.00 as they are enjoying the unlocked devices on it ? Well there are few issues with the iPad baseband installed on the 3GS devices and GPS not working is one of the main issue. This is where the dev team is working right now and looks like they are very close this team for real. They will be able to downgrade 6.15 to baseband 05.13.04 or earlier version which can be unlocked without any issues.
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