I think to make this binding and avoid issues further down the line you should get the tenant's signature.
My recommended route is to go for a new tenancy agreement with the new tenants. Although, you could proceed by way of addendum but I would be concerned as to whether this is binding on the parties.
Can you email a signature page of the addendum (or signature page of the new Ast if you go down that route), the tenant can print, sign and return. Once you receive the signed page back from the tenant you can complete.
Do you have further questions or queries?
It must be posted.
You can send a duplicate for the landlord to sign and return the duplicate as an acknowledgement of receipt.
The recorded delivery receipt will also evidence posting, provided that they accept delivery. That is why it will be best to post early.
If you are happy to proceed with online signing then that may be satisfactory and easiest method. I still think a new tenancy agreement would be best. Much better from your perspective as landlord. At least you know with certainty all tenants are on the hook liability wise.