Hi @AshishDas ,
Once the ownership is transferred, the db value for the account owner will be changed and the email template you choose in the user update rule for transfer ownership action will intimate the new owner regarding the ownership transfer.
Hence as per the design it is not possible to send an email to old owner once the ownership is transferred.
The request will be initiated via 'Manage Service Account' tile. Wherein, a update account request is to be initiated to by changing the owner from manage service account tile. When the request is submitted or completed. An email should go to the old manager stating that a new manager has been selected.
P.S : Approval request goes to the new manager.