Is this definitely the case?

I've just booked an economy ticket LGW->AMS->LHR using the "Lowest price (earning reduced tier points)" option. The outbound is booked in selling class O and I am being offered a £60 upgrade (on ba.com) in that direction. I phone BA to enquire about tier points on the upgrade and they said it would go into class U and receive no tier points.

I'm quite prepared to believe that the agent I spoke to was incorrect, but would want to be reasonably sure that I'd get tier points (since I have lounge access anyway) before paying for the upgrade.