Subrscriptions Clarity

[KTTT]

I need some clarity on Monthly Subscriptions.

I'm a Massage Therapist and I have some clients that prefer to be invoiced so that they can "pick and choose" their preferred payment. (Example, one month or several, they might elect to pay with a particular card, but might elect to use a different card due lack of available funds or "credit" on a specific card.)

Currently, I only have one client that prefers to be invoiced rather than have it set up to be automatically charged to a specific card on file.

It appears however, that each time the client receives the invoice and makes payment...the payment plan "setting" automatically gets changed from "Invoiced monthly" to "card on file".

Is this because the Client has - in the past - or during checkout when she pays the invoice, puts a check-mark (selects) the "save payment method" to file?

In essence, what's the difference between selecting "save payment method to file" during checkout for non Subscription clients, and Subscription clients??? Because I know there's suppose to be a way that Subscription clients "can" change or "update" how their subscription plan is routinely paid (invoiced or auto-paid via card or ACH, etc.)

THIS is frustrating me.

Thanks!