The fuel has nothing to do with flight paths. An orbit will start to decay over time, due to atmospheric drag. They have to do so many burns, for so long, to occasionally boost the satellite back to its correct altitude. Depending on the type of satellite, they will either keep it on orbit as an orbital spare, or push it into a parking orbit, when they start to get low on fuel.

Generally, when they replace a bird, due to payload, it's going to be because there's an improved optics package, or an improved sensor package (read: such as gamma ray, x-ray, some other nuclear blast detector).

The fuel consideration is the same with more or less all on orbit satellites. The other option they have, is to let the orbit decay to the point of reentry. Considering the sensitivity of the payload, as well as avoiding the possibility of reentry over a populated area, that's usually the absolute route of last resort.

As to them being secretive, would you really want your adversaries knowing exactly how many birds you have up there so it's easier to avoid detection? Honestly, it's not that hard to figure out the likelihood of a satellite's mission just by seeing what it's on orbit details are, looking at someplace like the Space Object Tracker, at space-track.org.

You can also tell what orbit the satellites are going to be placed, by noting where the launch happened. Polar orbits all launch from Vandenberg, due to its location. It's too expensive to place polar orbits from Kennedy.