This has always been my experience also. And as we all know, NO changes whatsoever on star awards (that involve carriers other than UA) after the first flight is flown.

Also - beware that doing this for example to change your outbound flights COULD cause your return flights to disappear if award space is not available at the time you make this change. You need to find a good agent who can handle this right.

FWIW, I share the frustration with UA's insane holding back on business & first class award tickets. At a time that we constantly hear that UA's biggest revenue problem is the lack of people buying premium fares, why is it that we have such poor availability in premium cabins? Baffling. These days, AA's premium award availability is leaps and bounds ahead of UA's. If however you're redeeming awards on UA partners, this seems to be pretty good (as shown in this thread and in my experiences), and UA's seems to get a LOT better as the flight date approaches.