does this mean I can delete the 'migrated photos to open up room

If you have migrated an iPhoto Library or Aperture Library to Photos and the items from the old library are all in your current Photos Library, you can remove the old , migrated library. The new Photos Library does not need the file. I would keep a copy of the old library on an external volume, just in case you should notice that photos or videos in your new library can to be read or metadata have not been migrated correctly.

You may not get as much fee storage back as you are expecting. Identical photos in the old library and the new Photos Library are sharing the disk space and as long as an image is still in the new library, the storage will not be released.

See: https://web.archive.org/web/20151125234013/https://support.apple.com/en-us/HT204476