Book something other than the cheap fare and they should be available should they not? The automatic upgrades are shown on the site when you book. Other airlines aren't any different. They hold F inventory as well.

In terms of UA, just remember that even at 75k, their benefits are limited. They only give you a 75% bonus and you don't get the 50k bonus. It's not until 100k where you get the SWU's which is the real value of their program. Depending on where you fly, you might be connecting through SF a lot. If you're based in Seattle or Portland, changing to UA is a hard thing to do particularly if you're just flying on the west coast.

UA have been trying to devalue all of programs for FF's across the industry. I wouldn't be in too much of a rush to change to their program.