In V4.2.1, a non-client can be changed to a client using 'Edit Party'
Ensure that a Client Team is set up, then use the drop-down to select 'Client'.
Once 'Client' has been selected, 'Save & New Matter' will become available, along with 'Save & Close'
For versions of OnePractice before V4.2.1, a non-client can be changed to a client by opening a matter for that non-client.
Ensure that a Client Team is set up
Select 'New' under 'Matter' on the ribbon, complete the matter description etc as usual, Save & Close
If the matter is not required, it can simply be archived