If I’m really lazy, the beers are similar, and the keg hasn’t been sitting around a long time, I’ll refill a used keg.
Usually I wait till I have a bunch of empties them clean them all. I have one keg full of Star San all the time. When I clean the others I push it to one of the freshly cleaned ones with CO2 and leave pressurized and empty. Doing this leaves me with a sanitized, purged keg. I continue to push from one keg to another until the Star Sam ends up in the last keg.