Create a much more robust automatic upgrade from the old client to the new client as well as stop distributing the old client with O365.
I have had several occasions where the old client stopped functioning, came back after the new one was installed and working fine and this is unacceptable. The process for clearing out the old and bringing in the new should be much more straight forward and active versus passive. In addition, the Office 365 suite that users download and install when they first log in should NOT have the old client included since a new client is out and should be the one that is used from now on per Microsoft guidance. Distributing software that is known to not function properly when there is a replacement for it is unprofessional and irresponsible.