I just received confirmed UDU/CPU upgrade to First on SFO-LAS for a flight tomorrow. I've done this route regularly and upgrades have been pretty reliable last year as a 1P, and thus far this year now as Platinum. My flight is currently showing Y9 R2, and the next three flights are all at or nearly Y9 R9.

This is the first time I'll be doing this route with a lap child. I expect First might fill up, but Y is still showing about 20 empty seats. Frankly I'd rather downgrade to economy if it is not totally full:
-to have an empty seat to use my carseat for my lap child, instead of holding her the whole way in F.
-to not disturb the elites in a full F.

Have people done this before, and if so, what is the best strategy?
-Basically do I request at the gate, since before then the agents won't be sure how full the flight will be and whether there will be two adjacent seats in Y?
-I've seen horror stories here where elites tried to pass an upgraded seat to a non-elite companion, and instead snippy gate agents took the upgrade away and gave it to someone else completely. Is there a polite way to request downgrade? I don't end up offending the gate agents and end up back in Y but still holding the child the entire flight.

EDIT:
-Oh, and in another thread someone said the pmCO system allowed only ONE lap child per CABIN. Aside from not making any sense (I was on a UA 777 over the holidays that must've had at least 12 infants - I suspect many of them didn't have paid seats, although we were the only lap child in F). If this is the new rule should I stay in F as I'll likely have the only lap child in that cabin?