I believe your telling me what you were told, trust me I worked with plenty of RSC’s who didn’t know their way around OPUS very well at all.
Your theory about B&M vs online also doesn’t apply, I’ve physically changed people from 24 to 30 and vice versa last year on iPhone X’s they pre ordered online. ANY time a phone is shipped, whether it be placed online or in store, and whether it was a pre order or an order for an existing device, opus handles it the same way.
If you want to test this theory you can even if the rep doesn’t know how to do it, simply walk into the store with a phone you purchased/activated within the last 14 days and ask if they can change the installment plan, if they say no just tell them to pretend like they’re returning the device and hit the return button and on the very next screen before you even get into the thick of it it will ask the rep “device exchange” or “contract-only exchange” and then once then on the next screen it will have radio buttons for whatever the current installment plans/agreements ATT offers(30,24,no contract, etc)