Modifying keywords without changing existing onese

Is there a way of adjusting the keywords on a selection of files without changing existing ones ?
For example I tag a set of movies I want to show in a playlist, say HomersMovies
If I then want to tag a different set say BartsMovies and some are tagged already with HomersMovies then it will remove the HomersMovies tag (unless of course I select only those that do not already have the Homers keyword to add the Barts one, then select any of the ones that are tagged as Homers and add the Bart to those too).
This is the only way I can find to do this - but it can be tricky once there are severl keywords on a file !

I see that when I select a set of files that do not have all the same keywords the Keywords field is empty.
So I was thinking what if I could enter a + in front of the keyword im trying to add when ive selected some files.
It would then ADD the keyword to those selected items but without changing what was there.
Similarly you could have it check for a - keyword prefix to remove only the given tag on a selection ?

You can put a comma next to the existing keyword and add the new one after it. HomersMovies, BartsMovies

Problem there is I dont necessarily know which it already has.
Also they may differ, so one file might be tagged only Homer, one might be tagged only Bart and I want to add Marge.
I dont want to add Homer if he isnt already there IYSWIM

If you add a tag that is already there then there will be no problem, the tag will not be added again if it exists.

But it will remove an existing tag.
So:
tag file 1 Homer
tag file 2 Marge
highlight 1 and 2 and try to ADD Bart so you would end up with
file 1 Homer, Bart
file 2 Marge, Bart
but you dont. They both change to Bart only.

I have reproduced this here. We will investigate adding support for the ability to add a keyword without removing existing keywords.

Great. Thanks