Which system version is running on your Mac?
If you already have upgraded to the Mac to macOS 13 Ventura you can no longer migrate iPhoto Libraries to Photos at all.
The only thing you could do on Ventura would be to import the iPhoto Library into a Photos Library, either your existing main Photos Library or create a new, empty library.
Change the suffix of the iPhotos library as recommended by Yer_Man from .migratedphotolibrary to .photolibrary, then launch Photos and use the command "File > Import" and select the iPhoto Library. Click "Check for Import". Photos will now show you all items in the iPhoto Library that are not already in your current library. Select the ones you want to keep, then click "Import selected" and wait for the Import to finish.
Then you can remove the older library or save it on an external volume.