Existing tenant moves to another property early

Other

I have a married couple who have a tenancy of one of my properties. They wish to upgrade to one of my larger properties.; They are still within the first 6 months of the tenancy. They are amazingly good tenants - as most of my retired ones are! and have redecorated the existing accommodation and re carpeted too. They want to do the same to the new place which is quite large and currently empty. SO - I want to give them a key for them to have early access until such time as I find a tenant for their original place, when they will ten move into the new one. The early access will not be charged as I take the view that once the old place is let that balances out the void situation. So how do I arrange the appropriate paperwork please?