Upgrades are free for Platinums.

On the day of departure. All Delta Skymiles members of all status levels are processed first - starting several days out for some of those Skymiles members - then all Aeromexico status members are processed next. If there are any seats left over after processing all DL and AM elites, then Flying Blue status members - together with the status members of all other Skyteam members - are in contention for whatever seats are left over and Flying Blue and other Skyteam elites will only have any such upgrades processed at the gate immediately before boarding commences (so: don't arrive late at the gate! If you're called and you're not there, you'll lose your upgrade)

But I wouldn't count on getting a free upgrade because there are so many other ellites that will "cut" the line and be placed ahead of you in the upgrade list, which you can see in the Delta app (and on the screens at the boarding gate). Unless you are lucky to be on a flight that is nearly empty and/or that has practically no other elite members on board, there simply will be too many other people ahead of you in the upgrade ranking that you have little or no chance of getting an upgrade.

I went from being something like being #4 on the upgrade list on the morning of my last Delta flight, to being #32 - and dead last - on the upgrade list at boarding time, which is the only time when you, as a Flying Blue member, could possibly be given an upgrade. Needless to say, I did not get the upgrade. But it wasn't so bad. I actually had an entire row to myself a couple of rows back from the Premium Economy (whatever name they use, I forget) cabin, which had every seat taken. At least I could spread out. I actually preferred that, than having a middle seat in a packed Premium Economy cabin.