Upgrading 3g to 4g with sim card, how can I do it?

[Joanna31]

Upgrading 3g to 4g woth sim card

I am new to this so here I go.....I have a galaxy s3 though straight talk I got it a month or so ago and I didn't get the Sim card kit just the minutes for for it so i added and activate the phone, now I would like to upgrade to 4g BC the 3g is slow, I want to buy a Sim card but when I called to fin d out it I could just move my plan and number onto a Sim with the same phone she said the Sim card wouldn't work(be comparable) with my phone? Is this true how can I upgrade to 4g? Hhheeelllll