Thank you Rudi and Valerie too for your comments - yes, I think it would make a lot of sense if one could bulk edit keywords without altering anything else... but I have another related question - does it help if one has many keywords, or is it better to stick to fewer keywords per image?
From reading the instructions here I got the impression that more would be better, yet yesterday, a web designer asked me (regarding website keywords) if I didn't know that each web page should have only 3 - 4 keywords, as more in fact reduces search effectiveness.
I was surprised, I didn't know that - and would like to ask if anyone knows whether the system here is essentially different than ordinary website search in this regard (maybe I ought to bulk edit all my images to "delete most"??? :-) )