How to replace a tag with another tag system-wide?
I've had two 'Green' tags, though with slightly different names for quite some time, so I thought I'd see if I could find a workaround.

Best I can come up with is in effect a find, add, remove.
Fortunately it turns out I'd only a few 'other Green' so it was quite quick, but you can almost see the Finder 'thinking' as it ticks through them all.

From Finder Prefs>Tags, make sure both are ticked &amp; then drop both your 'Greens' into the favourites at the bottom, circled.  You can only have 5 at once, so others will be removed. You can re-add afterwards. Leave this window open, you'll be coming back to it.



Open a Finder window &amp; select your 'to be deleted' tag in the sidebar, to list all the files tagged with it. Mine was called 'In Queue'



 Cmd ⌘   A  to Select All.
Right click &amp; assign the other green tag to the files, so they now have both tags.
Give the Finder time to finish - you can see it adding the second green dot…

Then - right click again &amp; remove your unwanted tag.

Everything is now tagged with the single, wanted, Green tag.

Go back to Finder prefs &amp; you can delete the extra tag altogether.