To cancel my monthly prepaid and change it as postpaid

Switching from prepaid to postpaid isn't as simple as just changing your plan; they're basically two separate systems.

Here's what you need to do: Log into your Self Serve account, go to "Change plans," and you'll see postpaid options available. You'll need to pass a credit check and select a postpaid plan to proceed.

The catch: you'll get a new SIM card shipped to you, and you'll receive a temporary number first. After that activates, you can port your current number over.

If your prepaid plan was recently charged, you won't get a refund when you switch. So timing it at the end of your billing cycle helps avoid wasting money.

The whole process takes a bit since you're waiting for the SIM to arrive, but it's doable entirely through Self Serve without calling anyone.

Need help with anything specific about the postpaid plans themselves?

You may want to check if you are eligible for the Fast Switch Self Serve Option. That would make it much easier for you to switch to a postpaid plan without having to go through the regular porting process. https://www.koodomobile.com/en/help/moving-prepaid-postpaid-monthly-service

Everything will stay the same, even your SIM card, payment method, and Self-Serve account will come with you during the transfer.

I would wait for black Friday to see the available deals. 

It may be better to set up a new postpaid line then port your prepaid number over.