android 4.1 to 4.2 upgrade for unlocked S3 Mobile

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http://androidforums.com – Hi Everyone, i have a T-Mobile Unlocked S3 Phone using in Singapore.i want to upgrade android from 4.1 to 4.2 . i have some doubt like after up-gradation my phone will be locked or i can use the phone in same condition with upgraded software? If it will be locked then how to upgrade phone in future ? want yours valuable suggestion.:) thanx (General)