I’ve got search&replace …
But no, that’s obviously the worst option.
Somehow I keep overlooking the mapping options.
(it might have to do with the colours 
)
I gave the mapping option a quick try, but I noticed two things that hinder me/I don’t understand well.
When I click on the ‘Source from:’ button in the Tag mapping tab, it becomes a darker grey and you can enter something in the field next to it.
But when I re-open that panel, it’s greyed-out again.
It doesn’t seem to hinder the purpose, but it looks/feels awkward, and I don’t think I’m able to tell if a mapping rule is active or not?
(maybe it’s a Windows graphics thing?)
Using mapping, it now writes both ‘show work movement’ and ‘showmovement’ tags.
I believe you can prevent that by adding an entry under ‘Initial tag processing’, but I have that part of the pane ‘greyed out’ and cannot enter anything in it.
I found out that it’s probably because I currently have ‘Create extra artist metadata’ unchecked in another tab.
But I wouldn’t guess that that setting on the ‘Artists’ tab would be of influence on the tagging options for the mapping panel?
P.s.
F.y.i. (or others interested), this little script works fine for this purpose:
$if($gte($get(_cwp_part_levels),1),$set(showmovement,1),)
$unset(show work movement)