If a major change is to be made, it has to consider / link to the old terminology,at least temporarily. If ever you changed these subject headings, I think a'hidden link' should remain, so that the search results would not be limited (i.e., ifI still use an old terminology, I don't want to be penalized).
makes sense, in order to keep things accessible to others
For the presentpurposes, however, we may take the general position that texts of all kinds have arelatively permanent aboutness, but a variable number of meaning(s)
A subject for a given text have a strong tendency to remain the same, whereas the purposes for which a text is acquired by a library or user may be numerous. For instance, a meteorologist may be looking at a document containing climate data to make short term analyses. A climatologist, by contrast, could employ long term analyses with the same data.