If a player wants to retire a character an play something else, it really isn't much of a problem. Typically, it means they aren't happy with what they are playing and want to try something else. It's all about fun, so why not let them switch? I may put in a minor story requirement, like they have to wait until they get out of a dungeon or to the next city to make the switch. Then on with the game. I always keep all my players at the same level when switching characters too, either from player choice or death. It makes balancing encounters easier. Plus what's the value in punishing a player with a lower level just because they were't having fun or the dice were brutal one evening? Most players go with a character they want to try the most. If it isn't what they expected, they may want to switch. Otherwise, I wouldn't worry about having a revolving door of characters for your adventures. People play what they want to play.
This situation may lead to some interesting character ideas too. Especially if both the player who's character died and the player switching want to work on characters together. I also let people know that if they retire a character, that character is still in the world and they become an NPC. They may cross paths again. Or the player may want to give that retired character a second chance.
This situation may lead to some interesting character ideas too. Especially if both the player who's character died and the player switching want to work on characters together. I also let people know that if they retire a character, that character is still in the world and they become an NPC. They may cross paths again. Or the player may want to give that retired character a second chance.