You must make sure you are acting within the scope of your retainer and that you advise on every area that is within such scope. This is a very high test and often the first thing that a client will look at if considering a negligence claim against you. For that reason, it is good practice to keep the scope of the engagement under review and update it where necessary. In many matters, this will mean amending the scope set out in the Engagement Letter as the client's requirements evolve.
You can amend the scope of the retainer by email. Do be clear as to when these changes take effect. Your email should note that the client has agreed to the changes set out therein, but where you wish to be more formal (e.g. where it has been some months since we last undertook work on the matter), then it is a good idea to use the Refresher Engagement Letter available through Keyed-In. Whichever method is preferred, it should address the following points as a minimum:
It is much more complex to deal with new clients coming onboard. Where the new client was always a client, just not identified as such, subject to the usual client set up checks (AML and conflicts), you can add them to the matter. Just ask the Clients Team to do this and have the client accede expressly to the Engagement Letter noting it should be treated as a party from the beginning. Note that this will mean the new client co-owns the entire file and is jointly and severally responsible for historic unpaid fees.
However, where the new client is to be treated as a client from that moment on, then you will need to bill out the current client/matter and set up a new one from scratch.