YB(M) upgrades don’t show up as ‘upgrade eligible’ or requested for me ever. Now, that is as a 1K waiting for PN to open - not sure how that is for Platinum. Once PN is available it shows ‘Premier Instant Upgrade Fare’ and I can click that and confirm - I believe that only works for GS/1K though.
YM(B) upgrades work on all CPU-eligible routes, the agent was mistaken.