Go to Koodo/Bestbuy/whatever, get a new number (new account) and the cheapest in-province plan you can get. This’ll only work in AB/BC/ON/NS if I recall correctly.
Go home, create a new self serve account with this number, go change your number to a MB or SK one.
Wait 4-5 days for the temporary billing cycle to roll over. Once you’re in the new billing cycle, change to the $48 5gb plan.
Wait another 24h for the change to go through. At this point, you’ve got your current number with whatever provider, and your MB/SK number with the $48 plan on the new SIM you got from the rep.
Go to change a number again, and port in your other number into the MB/SK account. It may say you have to wait 90 days, but you don’t – porting a number can be done anytime.
This will automatically close your other account with whoever, and port in your old number, overwriting your MB/SK number, letting you keep the $48 5GB plan.