iRacing

You cannot withdraw from any subsession (hosted, race, practice, etc.) and rejoin with a different car (or with the same car).

The reason for this is a single member can't have multiple results in the same subsession.

 
If you'd like to try out different cars in a practice session in a series with multiple cars, just select a practice session in another location/ region.

An exception would be if it is a Team race where you can withdraw and rejoin your same team (you can not withdraw from your team and join another team in the same session).