Search Replace Transfer
Transfer copies the contents of one field to another and gives you the choice of overwriting the existing text in the Copy To field, or adding to the Copy From and Copy To fields.
Here for example I want to add the filename to the end of the caption field and separate them with a hyphen.
WHICH IMAGES ARE AFFECTED?
The plug-in will target images which are selected. If no images are selected, the menu commands will be greyed out. I think it makes sense to force the user to deliberately choose which pictures should be affected.
REVIEW YOUR BACKUP
This plug-in makes it easy for you to change many images' metadata in a few seconds. Inevitably, that brings some risks.
As with all Lightroom work, do not rely on XMP sidecars, if you save them. Make sure you frequently back up your catalogue - daily, at least.