How To Geek:

• “You can transfer your phone number to a new carrier, even when switching from prepaid to postpaid plans. This process is usually completed online or in-store.
• Not all phone number transfers are successful. Some reasons include incompatible area codes, carrier limitations, and waivers from public utility companies.
• You can keep your old phone number when switching carriers, even if you don’t buy a new phone from the new carrier. You might need to unlock your old phone first.

Switching phone carriers is a great way to score a cheaper or more cost-effective plan. But the fear of getting a new phone number often outweighs the promise of a better deal. Thankfully, you can usually transfer your number when you switch carriers, even if you’re leaving a pre-paid service.”