Upgrading to UC from PE on an S fare code

Morning all,
I have a quick question regarding upgrades... My company, now tightening its belts, while still giving us the freedom to choose airlines, we must now only travel business if the flight is overnight or over 10 hours.
Anyway, my outgoing flight is business and the return is Premium Econ. I have a ton of airmiles, so thought I might as well try and use these to upgrade the return leg. I called FC and she said there was availability, took some details, then said she can't do it because it's a restricted ticket. When I stated that it should be a flexible ticket, she then said "oh, well it won't let me do it - I think they can only do it at the airport on the day".
That just seems a bit odd (and annoying, since there is reward availability for once). The fare code is showing as S. Does anyone know what the deal is here?
I have a quick question regarding upgrades... My company, now tightening its belts, while still giving us the freedom to choose airlines, we must now only travel business if the flight is overnight or over 10 hours.

Anyway, my outgoing flight is business and the return is Premium Econ. I have a ton of airmiles, so thought I might as well try and use these to upgrade the return leg. I called FC and she said there was availability, took some details, then said she can't do it because it's a restricted ticket. When I stated that it should be a flexible ticket, she then said "oh, well it won't let me do it - I think they can only do it at the airport on the day".
That just seems a bit odd (and annoying, since there is reward availability for once). The fare code is showing as S. Does anyone know what the deal is here?