Bulk mode in the Metadata editor: Search and replace
Assumes all are like this: Title : Isaac Asimov@Pebble in the Sky
Not tested REGEX MODE use the right side to see all is good
2 Passes. The first copies the Author to its proper place
Search Title:
Destination field AUTHOR:
Replace with: \1
The 2nd cleans up
Same search
Destination field: Title
Replace with: \2