Prepaid won't give you international roaming.
You can change to the $3/mo plan at any point (e.g. "I just want to keep my number"), then change to whatever plan you need while you're here. The "problem" is you risk not being able to get back to any plan that works well with you (e.g. if they discontinue the $3/mo, or if they discontinue whatever awesome plan you like during your visits). But, that's a risk whatever you do, unless you want to always pay whatever rate you have.
As long as you keep the plan loaded and paid for, you won't lose your number. As a bonus, you should be able to connect to WiFi Calling while you're abroad and use the $3/mo minutes for contacts you may need.
There's also a "cell phone number parking" service out there that you can port a number out to while you're abroad. I don't know what fees it is, but there was a post at some point with someone inquiring whether a number could be ported back from there. That may give you an option to save your number for a couple dollars cheaper, but may have a bit more hassle to move back to T-Mobile in time for your arrival.