IMHO just a popup message appearing right after a new payment option has been added, reminding the user to switch the payment method IF NEEDED, would already do the job!
Just consider the following 3 scenarios:
1.) SBB customer without Halbtax+ adds another “backup” payment method (e.g. other card) for additional peace of mind. E.g. in case the primary card (or reka-pay account) would be blocked for whatever reason while rushing, late as usual, to the commute early in the morning, so the ticket purchase or EasyRide activation needs to happen within seconds… (Been there, done that)
So after adding such a backup payment option, I would hate if the app would switch to that new payment method automatically without asking!
2.) Some companies require their employees to strictly use their company credit card for business travel according to their expense regulations. If using private payment methods, the expense refund may be refused. The employee would still use the Halbtax+ credit for private trips or for commuting to work. (Been there, done that)
3.) Halbtax+ is only allowed for personal tickets, while the SBB app allows “easier than before” purchasing tickets for co-passengers, which means the HT+ credit must not be used here. So the user has to learn switching payment methods.
That’s why I think that making users clearly aware that they must switch the payment method manually when required, is probably a better idea than whatever unnoticed / hidden payment method switching automatisms.