It is not news that you get different answers from different reps. Although what avek00 said makes more sense.

Once I was booked on KL JFK-AMS and I called to check on uprade availability before ticketing. I was told that no upgrades available on the KL flight, however, it is available on NW flight (codeshare of the same flight). I rebooked on NW ticket and got the upgrade. That was more than a year ago though. They might have changed their policy.