Say I have two tags, "Green", and "Green 1".
How do I replace all instances of the "Green 1" tag with the "Green" tag, across all files on my device?
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 & 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 & 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 & 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 & remove your unwanted tag.
Everything is now tagged with the single, wanted, Green tag.
Go back to Finder prefs & you can delete the extra tag altogether.