I would keep a backup copy of the migrated library on an external volume, just in case you should later notice problems in the new Photos library. The problem is, that Photos for Mac does not support all older image and video formats, that we could use in iPhoto, and you may need to convert some older items to a new format. You can open this library again, if you change the filename extension from ".migratedphotolibrary" back to ".photolibrary". The originals in your old iPhoto Library and the new Photos Library are indeed linked are sharing the storage by hard links, so you will not free much storage by deleting the iPhoto Library, but both libraries can work, if the other is deleted. Hard links are smart and are keeping track of the number of references.