Hi there,

no problem, when you get the new sim you actually have to follow steps to replace your existing one to use it by following the steps outlined here in your self serve account 

https://www.koodomobile.com/en/help/koodo-sim-support

Now you don’t actually have to use the sim as it is just a replacement, you can easily just put your old one in the new phone and it will work fine and keep the new one as a spare until you actually need it. So either put your old sim that is still active in the phone or follow the steps that are outlined in the link to use the new one, and that’s it you should be good to go just make sure to turn the phone off and on again when you switch the sims to reconnect to the network