If you aren’t an Elisp programmer—or at least some sort of Lisp programmer—this post won’t make much sense to you. If you do write in Elisp/Lisp, it tells you how to sand down a tiny bump in your workflow. It falls under the category of a small itch that you ignore until you can’t stand it anymore.
Over at the Emacs reddit, sauntcartas recounts how he always ended up needing to switch between let and let*. The difference is subtle—it concerns whether or not any of the arguments depend on any of the others—but the need to switch comes up naturally, often right in the middle of writing the code. It doesn’t take a lot of effort to navigate back to the let and add or delete the *, of course, but it does provide a bit of friction, especially if you write a lot of Lisp.
Emacs to the rescue. A tiny bit Elisp looks for the enclosing let and toggles it between the two forms. It’s a minor itch to be sure but it’s another great example of how Emacs can make your editing easier. If you write in Lisp, you really should add this bit of Elisp to your init.el. Sauntcartas’ code even reindents things if necessary.