I believe you assign the number to the SIM, so yes you should be able to just assign your #, but why is it you need a new SIM
@Elsa_Zeta, you have a few choiceS. You can cut your sim card to fit ( there is lots of information about this on the net). You can get someone to cut it for you. You can order a sim on line from Public Mobile. You can get a replacement sim from a retailer like WOW mobile or Walmart (note that some Walmart locations won't sell you a sim without activating it on site.
Assigning your number to a new sim is a very simple process, I’ve done it many times myself. Please let us know how it goes.
@Elsa_Zeta a little off topic but pm has been selling triple punch Sims for a long time. Are you on a legacy plan or an in market plan? The base inmarket plans tend to be cheaper than the legasy plans after the recent legasy plan increases.