Commercial property sold and new agent. Does the tenancy contract need to be updated?

When a commercial property is sold and a new agent has been appointed to manage the tenant from the new owner, do contracts need to be updated to reflect the changes?

The agent has said, "nah, don't worry about it", but what does commercial law say? He has also argued a solicitor is required to create a new contract and they are not willing to pay for it. I would have thought at least the schedule would be updated.

Is a letter to the tenant from the old and new agents agreeing that a change has been made enough?

I am concerned the tenant will not have a leg to stand on if something "bad" was to happen and the new agent argues it's not their responsibility, as they're not in the contract.

What paperwork generally needs to happen in these sorts of situations?
I thought the tenancy agreement would be between you and the tenant, the agent is only managing that agreement and doing other admin related tasks to do with the property management.

I would just get them to send a letter to tenant to advise them that *insert new company* is now the managing agent for you/or who ever it is