I recently watched my way through the sci fi series Andromeda - something of an unbranded Star Trek with a dash of spirituality and a giant Marty Stu front and centre. 

The first three seasons are a good romp - while it's obvious that Marty Knows Best and he'll always triumph, the overarching storyline and the characters are interesting enough to make that forgivable. There's a half-hearted attempt at Real Science, which is more than can be said for most sci fi shows involving aliens, and while there are some giant We Can Never Use This Solution Again, Or We'll Run Out Of Plots Real Fast plot holes, the series as a whole generally hangs together. At season four it starts getting, well, weird. Character motivations run inexplicably counter to previous setups, and there are quite a few episodes where somebody spilled Plot all over the script and didn't clean up after themselves. Marty Stu becomes a Marty of multiversal proportions, and the constant knowledge that whatever's gone wrong, he not only predicted it but already had it in his plan starts to drag.

Then we get to season five. *cough*.

It's not that it's bad, though the episodes are certainly not on a par with the earlier ones.  It's more that it has nothing to do with why we started watching the show in the first place.

The series throughline is: an undisciplined but well-meaning crew zoom about space on a ship (the Andromeda Ascendant) that, though antique, is arguably the most powerful warship in the known universe. They're trying to rebuild the intergalactic alliance-cum-democracy that ruled in peace three hundred years ago (Marty was frozen in time from the moment it fell.) and in the meantime, they're protecting the weak and innocent from the strong and exploitative. Each member of the crew also has their own agenda, however, which may not always be in their friends' best interests - thus the source of a lot of the tension.

Seaons one through four march to this drum, and it works.

Season five has them stuck on a planet that outlawed technology, fending for themselves and barely caring a blip about each other. The Andromeda's dead in space, and what we have is essentially a western. Not just for an episode, but an entire season. All of the arcs - the major goals, the major impending disasters, even the we're-friends-but-I-have-some-plans-that-may-kill-you tension that was sustaining the previous four seasons is forgotten.

There wasn't a sixth season.

Now, there's nothing wrong with bringing new things into your story or a series. The trouble is when you throw out so many old things at the same time that what you have no longer feels like the same story.

If Andromeda had kept one of the four main ingredients - the setup (ship, in space, doing stuff), the goals (the alliance) the baddies or the character-dynamic, they could have kept the continuity. It would have been a big jump, but there would have been enough of the 'old' show there to carry us over into accepting the 'new' show. By cutting all four at once, they essentially made their final series a spinoff of itself. There was a chronological connection of events to lead from one to the other, but they felt like completely different shows. There was no story 'core' to carry the audience.

Big changes need careful planning - you can't throw everything up in their air all at once. Some of the old plates have to keep spinning, at least until the new ones are balanced. If you're making huge changes, keep something constant, or you might has well just start a completely new story.