If you find an open seat is available you can call UA and they will change the service class. I do not remember if this is possible once travel has commenced, but no problem before hand.

On *A itins you can switch cabins on UA metal and change *A flights to UA metal before and after departure.

However, you can't show up at the airport and expect a *A partner to place you in the cabin you paid for, because UA only paid them for the cabin you ticketed.