Anyway, oto cats are stressed pretty easily so I'd switch gradually. Remove the gravel from one section of the tank at a time, replacing with sand. This way you don't have to remove the fish . Make sure you wash the sand really well, the dust tends to cloud up the water.
Is the betta in his own tank? If so, you could remove him and switch that tank over all at once. For the others, I'd leave them in and switch over the course of a week or so.