Keeping an RPG Series Fresh

We have already read and discussed Frayed Knights' developer Jay Barson's thoughts on RPGs and their design in the past, so his latest post, which deals with how to keep RPG series fresh, shouldn't come as much of a surprise to our readers. Here's a snip:
How much needs to change? Is a CRPG using the same engine or same game rules for more than a sequel or two doomed to getting stale and boring? Do you have to change everything to keep it fresh? At what point do you screw up the series so the fans no longer feel like they are playing the same game?

I've played several game systems where the mechanics of the game were fine (not perfect, but fine) for many, many campaigns. The game didn't need to change, only the story and characters. The mechanics formed a platform, a foundation, that didn't need many changes to make an enjoyable game. Part of the reason we've still enjoyed Call of Cthulhu for so many years is that we started playing adventures that really departed from the traditional Lovecraft / Chaosium formula. So maybe it was more of a content problem than a case of purely content issue.

On the CRPG side, I think of Planescape: Torment. While it used the same engine and rules system as the Baldur's Gate and Icewind Dale series, it stood out as something very unique. Possibly too unique, as I understand sales of this critically acclaimed classic were never as good as either of its siblings.