I actually disagree with having it set after a certain point. I think that if your work is really well done, even a main/key/important/noteworthy character being replaced can be very powerful. Take Valkyria Chronicles as an example of powerful character change. At a certain point, one of your characters change. This is done in such a way as to compel the player to really have an emotional feel for the characters. It makes your attachment to the characters stronger rather than weaker.
Similarly, I think that as long as you have a compelling reason for the character change and the new characters are very attractive, it can really keep the player going.
As another thought, think of it like sequels. Essentially, instead of having a separate sequel, you would be having the sequels all in one game. As a lot of my games have visual novels as their inspiration, I write my games in chapter by chapter format. Essentially, One chapter could be about one character, then the 'sequel' to that chapter could be about another. Visual novels have done character swapping as well.
Really, the important thing is to ensure that you do not lower your game's value by swapping around. If swapping around makes it so that players feel that the characters are no longer attractive or the game is no longer interesting, then don't swap. If swapping around is done in a way that makes players want to play more for whatever reason (maybe your main character was kidnapped, killed, etc. and you want to rescue/avenge/etc.), then it is done well. If the characters keep making the players feel immersed, then it is done well.
It all depends on how well the game maker feels that they can manage it.