iPhone jailbroken iphone upgrade ios

[yousufrafique]

I have an iphone 3gs which was not unlocked when I purchased it. I unlocked it via redsnow. I wanted to upgrade the phone to ios6 but since it was locked and jailbroken, I was not able to do it.
Now I have lodged an unlock request to original carrier and they have given me instructions that my phone is cleared to be unlocked via itunes and that I must insert original sim that came with the phone. Whereas I don't have that original sim. My question is that is it ok to connect my phone to itunes and upgrade it. my phone will be upgraded or it could do any harm to my phone.

 

[Applejuiced]

Go to settings/general/about and tell us your modem firmware.
If its 06.15.00 DO NOT try to restore it or upgrade the firmware.
You will need to downgrade the baseband first so you don't get stuck.

 

[yousufrafique]

thanks Applejuiced for reply.

Yes my phone's modem firmware is 06.15.00.
should I upgrade firmware.
How to downgrade baseband

 

[Applejuiced]

Use redsnow to downgrade the baseband first.
Don't try to update iOS versions before you do that.

 

[Carlanga]

afaik you shouldn't need to have the sim card, but any sim card from that carrier should work, it doesn't have to be that specific one. Make sure you downgrade the BB first like AJ said