Switching from a 4g to 3g phone and then back ..weekly.. sim card?

If this has been asked already I apologize...I've seen different answers and I just want to get it straight.


If I have a 4g phone ( sim card) and want to go back to a 3g phone (I'd like to do this several times a week). How can I do this without requiring a new sim card? Before, between 3g phones, all I had to do was call *228 and that was that...can anyone explain the easy way to do this?

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