Generally, it's not usually a difficult reservation to get. However, I've noticed that the closer you get to the date you want to go, the harder it is to get something (especially the more popular meal times). Boma really has gotten a lot of great reviews in the last few years and has become very popular. It is our favorite restaurant at WDW and we go to it at least twice on every vacation.
I've never really kept track of the menu changes. I know they change it about 2 times a week (mainly it's the 'meat' selection and the soups). My son loves the butternut squash soup and was very upset one time that they didn't have it. Although, the waiter said that even though it's not out, they always have it in the kitchen and she brought him 3 bowls! Maybe if you call the restaurant directly they may give you some idea of the menu changes.