haven't found an existing issue for this so:

we have a custom field linked to ~ 300 projects.
now i want to remove the link to ~ 200 projects, but keep the rest.
i'd have remove every single link, and accept the removal in next dialog.

my ideas:

1. create an "remove from all projects" button
   simply add a button to e.g. bottom of list, so i can remove all links and re-link again to just the projects i need.

2. enable possibility to "re-select" new link list
   instead of selecting links from the projects list and adding new links,
   replace the project links.
   e.g.
   i have ~ 300, so i select only the ~ 100 i want to keep, select "link custom field", and now only my ~ 100 projects are linked to the custom field.