It should allow you to unless you’re a new customer or recently made account changes.
What sort of error(s) are you getting when you try?

I am not a new customer , I had ported the number in 2021. Now when I am trying to change my current number , it is showing my current number with a status completed , already ported.

You can definitely change your number in the Self-Serve portal. https://www.koodomobile.com/en/help/changing-your-phone-number

It might be a system glitch , tried several times. I have requested for a callback tomorrow to discuss with customer care. Let see what is blocking this option.

Have you tried clearing the cache and cookies from your browser? 

Yeah, all options tried already. But no luck