can you delete the old iPhoto Library?

Do you mean, if you can delete the iPhoto Library after creating a new Photos Library from it?

Yes, you can. The Photos Library does not need the old iPhoto Library. But check the new Photos Library thoroughly, before you delete the old iPhoto Library. You may want to keep a copy of the iPhoto Library on an external volume, just in case you should later notice items that have not been correctly copied over.