I have a CASE WHEN condition I need to implement based on the select value.

Lets say I have this:

UPDATE Client
 SET NoProvince=
  CASE
   WHEN (SELECT COALESCE((SELECT TOP 1 Invoice.AddUser FROM Invoice
    WHERE Invoice.NoClient=Client.Numero AND Invoice.NoItem=23
    ORDER BY Invoice.AI DESC),0))<>1 THEN AddUser ELSE 0
  END

Basically, I need to select the most recent record in the invoice table based on NoItem=23 condition. Then, based on the value collected, in this case Invoice.AddUser, I need to have a sub condition to only replace Client.NoProvince if Invoice.AddUser value is not 1.

How can I fine tune this update to support this?