Wondering about this too, and how does one do it.

have a rental at SAN later this month. Booked using CSR, to take advantage of their primary rental insurance. But assuming rental is uneventful, would like to switch to my Amex to pay so that I can take advantage of an Amex offer. This is a pay later rental.

can you do this on website/app at return? Or just prior. Usually I use express return (I think that’s what it’s called - where you just bring car back and go) - once in a while, I do have an agent check in when returning at airport return line, but my more recent experience is that agents aren’t around that much to even do a check in, and I just receive the email on my way home.