I bought the SIM Card but forgot to swithc it from another provider.

If you did not activate your plan yes
Since the transfer is done only during the plan activation.
If you need a referral code in order to have 25$ on your 3rd bill, this is mine K61CD to use during the plan activation, when you have the total with the taxes.

Remember to DO NOT close the other account, the transfert will do that

https://fizz.ca/en/faq/activation-how-do-i-transfer-my-current-phone-number-to-fizz

During the activation process, here’s how to keep your current phone number:

1. Click Transfer a number to Fizz.
2. Enter your phone number in the field that opens up below.
3. Use the drop-down to choose your previous provider.
4. Enter your previous provider’s account number or your phone’s IMEI# in the field below.
   • Find your account number on your bill.
   • 
     Dial *#06# on your phone to get your 15-digit IMEI number.
     • Only the IMEI phone number used at your current provider will allow the transfer.
     • If the number is more than 15 digits long, enter only the first 15 digits.
5. Click Next, review your first monthly plan payment and click Submit.