Does anyone know whether drains have multi purposes? I have an existing basement bathroom with a shower and a toilet. I want to increase the size of the room and replace the shower with a tub but move the tub to where the toilet is currently and put the toilet where the old shower is. Can I keep the same drains and retrofit the tub onto the toilet drain and the toilet onto the old shower drain?:confused:

You can make the shower fit where the toilet is but you can not make the toilet fit where the shower is. To move the toilet you would have to have at least a 3 inch line that does not reduce in size in any way. Plus you would have to have a vent for the toilet, some areas alow this to be a two inch vent.
To make the shower fit where the toilet is you would have to reduce the size to a two inch line and at a trap. The shower would use the vent that the old toilet did use.