Is the following scenario in Android technically possible?
Is this possible without the user confirming or triggering this change of payment card? The whole point of this is to quickly allow a secondary card to be used as payment alternative, without manually switching for the user.
I am guessing you are trying to make an app like described and you are not talking about an existing app.
This is only possible if you "are a bank". (clearification below)
Because paying with NFC on your phone is like paying directly per card. The merchend charges the given card (which is presented via NFC).
If you are a bank or have the ability to implement a system like the following it would be possible.
I do not know if the merchant will wait long enough for your backend.
Take a look at https://www.solarisbank.com/, they offer banking service for other companies and have a "straight forward API".