I just did this. It was painful and the outcome wasn't ideal.
I called T-Mobile to switch my plan from Ever More Plus to the Monthly4G plan. The good news is after more than 1 hour of talking to mostly Indian operators with really bad line quality, and getting hung up once in the middle, I was able to retain my old number. Also the left-over credit from the old plan was transferred to the new plan.
However, they couldn't give me the $30/100min/unlimited web plan, because that's "new activation/walmart only", and not available to me. I ended up using the $30/1500min/30MB plan, and see if I can really squeeze my web usage back to the prehistoric level.
You are more than welcome to keep your phone, sim card, AND number when migrating from flexpay to Monthy 4G service as far as the plan that fits you best you can take a look here
and gti2kids is correct the 30$/100min/unlimited web plan is a wal-mart promotion only.
Please don't hesitate if you have further questions.
Good question and the short answer is maybe, basically the process would work like this:
Go to walmart and request to activate a T-Moblie Monthy4g line (not a family mobile line)
You will get a different number
Cancel your current prepaid line
Call the support for prepaid (it is prepaid who handles the number change)
They will submit a form to request the old number back (takes about a day)
As you can see with this process your line will be canceled for a short amount of time (about 24 hours) so although we do try to recycle numbers for 30 days it doesn't always work that way and you have a slim chance of losing the number, but in personal expirience I have never seen it fail.