I accidentally joined House Hlaluu, but I would prefer Telvanni, and I already saved the game, so it's to late to do anything about it.

I know it's not possible to change in the game, but is it possible to change in the console? Is there a command that can remove me completely from House Hlaluu so that I could join Telvanni instead?

You can use the console to join all three houses if you wish. Not sure if you can leave them them though.

PCJoinFaction 'Telvanni' should do it.

There is always one quest where telvanni and hlaalu are in conflict (Odirinan or something like that). You can get round it if you bring the girl back to the Hlaalu reps, then kill the reps and then kill the wizard. It's a while since I played but it is possible.

The hard part comes when you have Strongholds. There is a mod that enables you to have all three strongholds though it conflicts with LGNPC and related mods. You can also do it by repeated use of the console (I know, I have).

I haven't found a command to leave a faction.

That solves my problem, thank you