My understanding is that it should automatically transfer over as long as you have a bill over $25. If it doesn't transfer over the code is still valid so you can re-add it.
If you go onto you public mobile app and look under your subscription tab, then go to manage subscription. It will tell you under current subscription.
Check your settings and make sure that you have a roaming enabled. Sometimes it takes a few minutes I've noticed when I go from the Canada into the US for to start working properly.
