upgrade jailbroken, unlocked iPhone 3gs with version 5.0 to 6.0 but without tethered boot

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Thanks in advance for any useful answers!

I have a 3GS which I emancipated (jailbreak, unlock) from ATT for use in the UK when I travel. I have a UK vodafon sim in it. I did the jailbreak, etc., myself. Thus, I have gone through this successfully at least once, but I am no expert by any stretch.

Right now the 3GS has version 5.0.1 (9A405) and modem firmware 06.15.00

I want to upgrade to 6.0 so that I can use iMessage.

From what I can tell from my reading, it seems that if I upgrade to 6.1.3, then I must have the 3GS connected to my computer each time I turn it on. That's tough to do when travelling.

I hope I am not misunderstanding what I am reading. But here it is...

Q: Is there a way to upgrade to some version of 6.0 that does not require the tethered reboot?

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