Changing from room only to single AST

Tenancy Types and Management

We have four tenants in what used to be our main home. In order to comply with the mortgage (which is one we do not wish to change if possible) we can only get consent to let by changing the four room-only ASTs to a single AST. This will require the tenants' permission of course, but as they seem happy there this should not be too difficult.

What procedures should we go through to process a single AST for four names?

Thanks