If the tags have been written to the pictures themselves, or to sidecar files, you can just move the pictures there and re-scan that new folder to your library, although that will take a while. If you want to make sure that the metadata is indeed saved, you can check using an external tool (like this website:
http://exif.regex.info/exif.cgi). If you want to manually force that metadata into the files, just to be sure, select all the relevant files and in the main menu go to Item/Write metadata to file.
I'm not sure if there's a way in digikam to "migrate" the location of the library...