There is a lot to unpack here.
First of all, "deep discount" doesn't refer to the price you paid, but the fare you bought (which is related but not equivalent to the booking class).
Did you have a look at this?:
https://www.flyertalk.com/forum/brit...l#post32910040

Ignoring what the BA agents said for a second, there are many cases in which a ticket is not eligible for an upgrade (see the thread I linked to).
Technically, all CW booking classes should be upgradable (unpublished/IT fares might be ineligible).
This, however, only applies to flights operated by BA, if you want to upgrade AA/IB flights on BA stock, the ticket must be issued in J/C/D.

Without further details on your routing, fares, etc it's very hard to tell what's really going on here.