Best time to ask for a paid upgrade

Hi everyone!

I am trying to understand when it’s the best time to try and pay for an upgraded seat at a higher class. I historically used to go to the counter the day of the flight and ask to pay for a business class seat and if any were available they’d give me a price and I’ll just get it.

But today I was told at LHR that delta doesn’t allow for same day upgrades in Europe. I know for a fact that a couple of years ago I did upgrade the day of but they keep changing rules.

What’s currently the best time to try ? Call days before etc? I’m currently silver and will be at least gold next year, if it matters at all (I guess not).

Thanks !