The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to Media Finder and try your luck there. Want to propose a new trope? You should be over at the Trope Launch Pad.
Substitution Tropes is the index for characters being replaced in general.
But to be a Trope, we need to know the narrative function, such as Changing of the Guard (protagonist retires/dies, is replaced by new protagonist who does the same thing), Old Hero, New Pals (protagonist stays the same but swaps out new comrades) or Passing the Torch (previous heroes retire/die and charge the next generation with upholding the same work).
Just "old characters leave, new characters enter", absent any other context, wouldn't be its own trope.

It can be by either putting them on a Bus, or by killing them off.