You can also have more than one device set up for TOTP - phone, previous phone or tablet, desktop using WinAuth or similar. Authy and the password managers will also track those seed values, though it's best to store them separately from your actual password storage.
Another thing that will not migrate phone to phone is Signal conversations if you're inclined to keep those.
Yes, I lost my Signal conversation history as well and had to be re-added to all group conversations (another PITA).
How do you store seed values in password managers? More specifically: how do you export them from Coogle Authenticator? (I’ve not found that option). And how do you import them again?
ISTR at least one service (AWS or Google) asking for the first 2 codes after scanning the QR code, probably to sync. So I always assumed simply re-scanning the QR code wouldn’t be enough but maybe it is this simple.
The underlying technology behind (almost?) all of these is TOTP (https://en.wikipedia.org/wiki/Time-based_One-time_Password_a...) which pretty much just depends on both systems having clocks that are reasonably close to synced. The initial value is basically a random number generator seed, and given the seed and a number of iterations (based on the time differential from a set starting point) calculating a code is fairly simple.
There would be problems on fully-isolated systems experiencing clock drift, but on any modern Internet-connected system using NTP or on any cell phone with time synced to the network it shouldn't be a factor. The most likely problem scenario is probably a corporate network using only an internal time source that drifts.
Doing a single code as validation only makes sense to catch transcription errors since in case of problems someone could end up locked out of an account.
Another thing that will not migrate phone to phone is Signal conversations if you're inclined to keep those.