I only upgrade BA via POUG, and at that it's nearly always for tier points on short flights so the corporate TA doesn't need to get involved. We have to buy all our flights through the corporate TA.

My view is that if I POUG I know it makes the ticket partly or wholly inflexible, and if I have some sort of emergency need to change my return travel then I will have to pay the cost of some other ticket myself. So far, in several years of doing this, it has not happened - and in any case the maximum liability for a trip from a large European city is manageable (in particular when an LCC is often an option). I am more careful about POUGing an intercontinental trip, but even there if you POUG a return trip shortly before travel you should have a good idea of how likely a need for a short-notice change will be in your job and your company.

On AA, I have used SWUs, but these don't change the underlying ticket like BA upgrades do so the ability of the TA to change the ticket is not affected.

I've never tried talking to our corporate TA about changing the ticket at my own cost. Given how much value they subtract in other dealings with them, I dread to think what they'd do if I asked them to handle an upgrade.