A new buffer to a new file path before first save with adjustment to
mode kind and comment start prefix convention triggers the suggestion to

 M-x revert-buffer

for change to take effect, which turns out to be incorrect for a
buffer never before saved to file.

 M-x normal-mode

does the job correctly.

Steps to recreate

 1. run, emacs -Q -nw
 2. load theme womat for readability, dark blue on black is unreadable on input buffer below mode line
 3. open blank buffer to file without saving, /tmp/x
 4. type at top of file, # This comment line is left intentionally blank. -*- prog -*-
 5. add prop line, M-x add-file-local-variable-prop-line
 6. enter, comment-start: , "# "
 7. *Messages* buffer says, For this change to take effect revisit file using M-x revert-buffer
 8. *Messages* says, revert-buffer-insert-file-contents--default-function: Cannot revert nonexistent file /tmp/x
 9. M-x normal-mode ; does the job suggested at 7. above