You need to do the following:
1- Purchase a PM SIM
2- Leave the old SIM in your phone
3- Activate your SIM on your PC or laptop.
4- Reply to the text from the old carrier with YES confirming you are porting over to PM within 90 minute window
5- Your old SIM will continue to work until the porting is completed
6- Power off the phone when the old SIM stops working and swap the SIM
7- Power your phone back on
Welcome to PM!