How? The original battery provides power while you're soldering on the backup and the backup is solidly soldered on while you swap out the original. How could you be so clumsy you rip out a solder joint?
What if it's only held in by the case itself and not a clip? Then I'd disconnect it simply by opening the device?
Since you knew it was a CR2032 I assumed you were already familiar with it.
I just guessed by the physical size of the device. I guess it could be a smaller coin cell as well.
You are correct. Those BankID physical devices have tamper switches that erase RAM on device opening.
An alternative is debit card bankid (as a smartcard, then add Mobile BankID back), but it's being deprecated soon.
Accidentally disconnecting the original battery while messing with it. Or shorting something out.