I would say can't be done.
There is a process called triangle swap that carriers have in the past done. It involves the carrier internally moving the phone numbers around to achieve the desired effect. But even if they are capable of doing it (and most carrier customer service representives either didn't know how to do it or weren't allowed to), good luck in getting the carrier agree to it. I believe I have also read that some carriers have made policies prohibibiting this, likely due to the technical nature of swapping 2 different phone numbers with each other's accounts.
As for trying to achive all this yourself: Once again, not possible - sorry.
Since you appear to have a desire to keep the plan with the 12GB of data per 90 days, you'll need to realize that once you port that phone number out to Koodo, you also lose that 12GB plan. You'll need to decide which you value more: that 12GB plan or the ability to take that phone number over to Koodo.
I believe that the closest you're going to get what you're asking for involves porting the phone number with the 12GB plan out to Koodo, but then just changing the plan on your remaining Public Mobile account. You would need to choose a plan that includes more than your current 4GB data, but unfortunately, you will need to choose from plans that Public Mobile currently offers.
