Andromeda was also a show that skipped out almost entirely on its initial premise in its final season.
Trek shows DO change, though, especially in their 3rd seasons. DS9 went from sociopolitical character studies to space battles and war in season 3. Enterprise went from aimless wandering to space battles and war in season 3 followed by finally showing the foundation of the Federation in season 4. TNG got good in season 3, as did Voyager. The biggest shift was in ENT, and I hope the STD shift will be as successful... because it really needs to come with serious A Game to make me not desperately miss Pike and Spock.
The 26th Century aka the 2500s.