Use the quickfix list.
Fills the quickfix list with matches.
Be sure and disable anything else which might stomp over the quickfix list,
like ALE. Or, use
:lgrep which fills the location list, which is scoped to a
Acting on the results
Run a command (search and replace, then write) for each file in the quickfix list:
:cfdo %s/whatever/whomstever/g | w