Thanks. So I shall set it up now, port later. "try before you buy" even!
You apply for the Birdie sim, at the time of application can choose a brand new number or to port your old number across. You also select an "Activation Date/Time" which is usually from 1am - 3am on any day ranging from 1 week from now to 3 months from now.
When porting numbers across, Birdie will warn you to make sure your old number is not still tied to a contract, and is in your same name as shown on your HKID card. If not, the porting will fail. This is why I said good luck in my previous post as I'm sure you'll probably have a few extra steps in their somewhere to get your number out of a company plan.