If no transactions have been created for the timesheets, you can change the primary client in Zoho Projects. Once this change is synchronized with Zoho Books, a new customer profile will be generated in Books, replacing the previous customer associated with the project and the original customer profile (synced as initial primary client) will still be retained in Zoho Books.
On the contrary, if transactions have already been generated for the timesheets, it is no longer possible to change the primary client.
Books
Online accounting software
for small businesses.