Lightroom can import/export keyword lists, so editing the hierarchy in a text file is the easiest way to reorganize a large tree. However, based on the answers, that approach is likely to break the existing links between photos and those keywords once you go outside Lightroom.

So the short answer is: there doesn’t appear to be a reliable Lightroom method to import a revised hierarchy and automatically preserve existing photo-to-keyword assignments in the way you want.

A possible partial workaround is a plugin that explicitly adds parent terms to images after you restructure the hierarchy, but that is not the same as safely consolidating/importing the hierarchy while keeping all existing links intact.

If you proceed, test on a small catalog first. In general, restructuring entirely outside Lightroom is easier for the keyword list itself, but preserving current image keyword associations is the risky part.