Or can I replace one material with another for all its users.
(I don’t want to modify it - which would change the appearance of all users - because I have already made the material elsewhere and it would be tedious to repeat all the settings)
Select all the objects you want to assign the material to, then select the object that has the material applied to it, and then ctrl+L and select materials. This will give all objects selected the same material as the last one selected.
Thank you,
and what happens if the material to be applied is in a slot for the user? being one of 3-4 materials found on diferent faces of the object ?