iphone 4 4.0.2 to 4.1?

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ok guys so here is the thing , i got the phone with 4.0.2 , i didn't update it , got it from the sim card provider at my place , i want to update to 4.1 , but scared cuz i saw people who said that after update the phone didn't recognize the sim anymore ,and another guy was saying that the greenpoison probably won't work with him cuz his baseband updated or something ,i rly don't understandthat stuff ,hope u guys can help me out ^^ . didn't register the phone yet btw (is it important ?) .