Forget about UA systems - most places won't change the payment method once its been charged. I had Wayfair do it for me a few weeks ago (placed original order several hours before my CSR card arrived, but they are really top-notch customer service anyways). But this is most definitely not the norm - issue I'd guess is that most systems aren't designed to do this - they can take payments, or refund them, but not much inbetween. Perhaps new modern systems have this kind of option, but I wouldn't call most airline systems 'modern' by any stretch. I am on PayPal as a small business as a merchant, and I don't believe there is a way for me to do that, either.

Also, UA systems (on the ground anyway, iPhones in the air are different) are very good at charging cards immediately. I don't think I've had a time that I can remember in many, many years where a ticket doesn't show up as a pending charge right away. My guess is it may be possible, if it is at all, between ticket purchase and ticket issue, but typically that window is quite small - a few minutes for all UA/UAX operated itineraries.