Notepad++ version 7.6.4 (I think) introduced the capability to rename an unsaved tab.

For example, a tab with the title "new 2" can be right-clicked upon, "Rename" chosen from the resulting popup menu, and the following box presents:

What is needed is a way for a plugin to be able to effect this sort of rename, directly, without the popup input box shown above.

[ Comments above based upon features available current to Notepad++ 7.9 ]