In order to properly transfer both rights and obligations under a contract – eg, the rights and obligations of the tenant under a lease – a mere “assignment” is not sufficient: an assignment can transfer rights, but not obligations.
To fully replace a party to a lease or other contract, it is necessary to enter into a tripartite agreement called a “Deed of Novation”, pursuant to which the incoming parties step into the place of the leaving parties, taking over not only their rights/benefits but also the burden of their obligations.
This creates a direct contractual relationship between the continuing parties (eg, the continuing lessors) and the incoming parties (eg, the new lessees).
Novations can be complex to document, especially where there are multiple continuing/leaving/incoming parties.
Additionally, there may be guarantors that need to be replaced or released, or new guarantors to be added.
If done manually, this can be very fiddly and time-consuming.