I think you'd be better off giving your solicitor a call first. I think you'll find they say don't touch the system if the buyer has already seen house. It'll just be a matter of including the usual boiler plate in the contract that says that the seller makes no warranties about any system and the buyer must commission their own surveys and tests, and rely solely on them.
Don't worry about providing 'IT Support' to the new owner. Don't offer this as part of the sale and once the sale has completed don't answer their calls.
Personally, I agree with this, homes are going smart now, be upfront about how it's a step forward.
Paul - If you really want to be proactive, spend that day you were going to spend re-wiring it carrying out normal tasks on it, like changing times, extending for half an hour and write out a cheat sheet for each one and give it to the new owner.