I cannot see the forest for the trees, so here is my dumb question:
"Your old SIM card must be inside your device and your account with your previous provider must still be active so you can receive and respond to the SMS and authorize the request..."
I bought a PM phone, and a SIM
I have my old phone with it's (old SIM); service still active
Do I have to install my old SIM into the new phone, or can I initiate porting from the old phone/old SIM?
After activating your Public Mobile service, you can try your Public Mobile SIM card out to make sure that your new account/service is working. However, other than for maybe a quick check of yor Public Mobiel services, you should re-insert your old carrier's SIM card. This is because the old carrier will send you a text message that you need to respond to that confirms for them that you really did ask for yoru number to be ported to Public Mobile.