Use bulk metadata search/replace. Assuming that your genre column has the type "comma-separated text, like tags...", use the following process.
- Make a backup of your library in case things go very wrong.
- Select all the books you wish to change
- Press 'E' to get bulk metadata edit
- Select the search/replace tab
- Search field: tags
- Search for: leave empty
- Replace with: leave empty
- Destination field: #genre
- Press Apply
- Click the "Basic metadata" tab
- Check the "Remove all" box next to "Remove tags"
- Press OK
All the tags will have be copied to #genre and deleted from tags.