

Salesforce customers/users receive the standard/default email every time a case is assigned to him/them. The template cannot be modified as the user will not have access to that email template.
A workaround for this is to create a brand new workflow with a custom email template.
Chang Case Owner Assignment Email Template in Salesforce
Disable the default functionality and create a workflow that imitates the default functionality but with a different email template.
To disable the default email functionality,

The case owner receives customized emails.
None.
There is no straightforward way of changing the email template of the “Change case owner” trigger. This is the easiest and optimal workaround.
Once a case is assigned to you or to your queue and a notification should be sent out, the related person(s) receive the following email: ———————————– subject: Case transferred to you. ———————————– Case 00001946 has been assigned to you. Please click on the link below to view the record. https://<instance>.salesforce.com/<id> ———————————– That is in general pretty handy, but users complain that they want to see the subject or other properties from the case. However, I learned that it is not possible to change that system-generated template. So the obvious idea is: Being is able to alter this system template. For example, set an existing “Email Template” in the position of the system default template. This should be a very small change in SFDC software that enrolls a great new feature for user customization.