It is best not to impose one carrier's scheme on another. OP was flying AA domestic in Y on what was likely a 2-class aircraft. While there are always other possible scenarios for UG's, the principle means for upgrades on AA domestic are elite status on AA (not OW). This is made crystal clear on AA's website.
Any AA status member may request an UG on the flight. If EXP or PLAT PRO, the UG is free. If PLAT or GOLD, it is paid for in 500-mile certificates (sometimes called "stickers") which are now electronic and which are earned in increments for other flying or may also be purchased for cash. This flight would have required 5 such certificates. But, OP is not an AA elite and accordingly is not eligible for this form of upgrade.
As to OPUP's and other issues, those are not what the conversation OP had with the agent at the FL were about.
AAdvantageŽ elite status members can upgrade from any eligible purchased, published fare in the Main Cabin (excluding Basic Economy fares) to the next class of service for travel within North America on eligible American Airlines marketed and operated flights. Executive Platinum members can also upgrade on AAdvantage award tickets.
Upgrades are always complimentary for Executive Platinum and Platinum Pro members. For Platinum and Gold members, upgrades on flights 500 miles or less are complimentary and flights greater than 500 miles require 500-mile upgrades.
If possible, upgrades will be confirmed before departure based on elite status.