I assumed you are a postpaid customer? If so, you can upgrade your phone with selfserve. Follow this https://www.koodomobile.com/en/help/upgrading-your-phone

Regarding issue with your current plan, you would have to switch if the system didn't let you keep it.

Buying directly from Apple is also a good choice. All you have to do is to put the Koodo sim in the new iPhone

I pay a monthly bill, I don’t know if that is postpaid? That shouldn’t be the case though. They do offer more than just the $33 and $60 options. I pay $45/ month and i am very happy with what that plan provides, I don’t need more or less. I do think they just don’t make it easy to figure that out. Was hoping someone had similar experience. 

If you’re buying the phone from apple then you don’t need to do anything with Koodo.

Just take the sim from your current phone and then put it in the new one.

Your plan will continue and you won’t have to change it.

Is it really that easy? I know the sim card size is the same, I just thought it was more technically complex I guess. Thank you!

I have a plan with Koodo and a Motorola G7.  I purchased a Motorola G Pure  in the USA from Boast Mobile and had a plan with them for two months.  Upon my return to Canada   I took the Koodo SIM card out of the G7 and put it into the Motorola Pure and the notice says Network Blocked.  I was able to connect with the Internet but no phone or text service.

Was your phone unlocked from the US carrier? If you saw the message network lock or blocked, the phone was likely locked to that network.

You can see Sophia’s reply to your question here @Bruce Towner 

So it really is as simple as swapping. That’s the good thing about sim cards. You can swap sim cards and phones anytime you want, wherever you go. What phone you own and by whom doesn’t matter, so long as it’s compatible with the network.