pre paid to post paid internationally

I have had the T-Mobile Prepaid Unlimited plan for years. Great plan, the problem with that now is that I am currently outside of the U.S and will be for a few months. I still need to receive calls from the U.S but my plan doesn't allow me to receive calls from everyone, only certain people that have a certain plan. 

I want to switch to the T mobile essentials plan due to the fact that it allows international calling. Is it possible to switch from prepaid to postpaid while outside of the country?