IOS – From a jailbreaked iPhone 3g 4.1 (foreign sim locked) to 4.2

A friend of mine bought an iPhone 3g with a sim card in another country. Some months ago he gave me this iPhone, but he forgot to give me the sim too, so I've jailbreaked it leaving the sim-lock, that is I don't use this iphone for the phone calls.

So now I have a jailbreaked iPhone 3G with iOS 4.1 and I would to pass to iOS 4.2

How should I do? If I simply upgrade from iTunes, will it ask me for active sim card?

Moreover, I've read that if there is a problem, it is no possible to downgrade to 4.1, isn't it?

EDIT

I've the baseband 05.14.02 and bootloader 5.9

I'm not interested to use this iphone for phone calls, but I would maintain the gps.

Is it possible to pass to 4.2.1 maintaining the baseband 05.14.02?