You can always switch to yallo and take your phone number with you, regardless of whether you have a plan or a prepaid SIM.
If you already have a plan and want to switch to yallo, you can always transfer (port) your number. Keep in mind, however, that you may have to pay extra charges to your current provider, depending on the contract duration, if you cancel before your contract expires.
If you have a prepaid SIM and want to take your phone number with you to yallo, you can always do that. Please remember that this process takes about ten days.
If you are already in the process of porting your number, you can look up the status here in my yallo at any time.