You can change your minutes. You can change other plan features (add or remove them). I've done this. The only thing you can't change if you want to keep your grandfathered, unlimited data is your data plan itself. This includes adding tethering through AT&T. I've confirmed this with AT&T. Friends of mine have also confirmed this on their own.
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You can change your minutes. You can change other plan features (add or remove them). I've done this. The only thing you can't change if you want to keep your grandfathered, unlimited data is your data plan itself. This includes adding tethering through AT&T. I've confirmed this with AT&T. Friends of mine have also confirmed this on their own.