Purchases & Returns

Wondering why the online koodo store keeps telling me to go to the nearest store to me?

I currently have an iPhone 6s, but I am planning to buy a new iPhone (maybe an 11 or 12) by the end of the year. I am also planning to buy the phone at an Apple store instead of going to my local Koodo retailer because I don’t know if buying at Koodo will change my plan. So I wanted to ask: if I were to just buy a new iPhone at an Apple store and not Koodo, will I be able to just switch out my SIM card with no problem? Will I also be able to keep my current plan if I were to just get a new phone? Thanks.