I've done that in the past (i.e. called in and spoke to an agent) where it was necessary to avoid the change fee, but I'd assume with the current climate you can just log into your booking on AA and then choose change and then choose the new cabin as part of the change.

I certainly wouldn't be doing a 24-hour cancellation and then rebooking. That would have all sorts of potential issues, especially where your accounting / expense system has feeds from your credit card company or travel management software. In this situation it's fairly reasonable to show / argue that it's an upgrade and you're paying the difference. Imagine the situation where you book a full fare, refundable ticket in Y, claim that on expenses and then put that in for a refund before booking a basic economy ticket instead. That's not quite so easy to explain