Yes, that’s how it works…. when you port your number from one provider to another, the “old” provider gets cancelled right away.

You cannot keep your number under two service providers. If you want to keep your old account, it would be best to sign up with Koodo and get an entirely new number, while maintaining your other account with your old service provider. 

Unless your have another line with your old carrier. This way the account will stay active but the line will still be canceled 

Get a different number with Koodo, sign up for Call Forwarding with both providers.  Turn that feature on/off at will. You can then receive calls placed  to both numbers on whichever line is the current target, and use either for outgoing.

Voicemail will be a little more complicated, with 2 line to check.