Yes, you can back date a Move Out on a tenanted property, however the portal will treat this as a "No Switch" move.

Adding a genuine Move Out instructs our energy partner to switch the energy supply at the property. Whereas a No Switch Move does not instruct an energy switch but allows you to notify the current provider (if known) the Council and the Water of the change of tenancy.

Importantly, when you are backdating a No Switch Move, you must follow the 1 step wizard all the way through the form until complete: meaning you must do the back dated Move Out and the new Move In at the same time. If you do the Move Out separately to the Move in, this will switch the energy as normal at the tenanted property, which may cause complications such as an Erroneous Transfer.