How do you get iphotos and photos libraries to migrate into one?

So that we're all on the same page let me restate your situation as I understand it:

You have a Photos library and an iPhoto library that you'd like to merge into one Photos library, right?

The iPhoto librarty isn't the one that was used to create your current Photos library is it?

There are two ways to join the two libraries into one. First migrate the iPhoto library to a Photos library. You now have two Photos libraries.

A - using the iCloud Photo Library feature. This will require purchasing enough iCloud Storage to house the joined library, at least temporarily.

1. 1. Enable the iCloud Photo in your first Photos library (A). Set it to be the System Photos Library.
   2. Let it upload all of the photos in the library to the iCloud Library.
   3. Set the second library(B) to be the System Photos Library and enable the iCloud Photos Library. Let it sync with the iCloud Photos library. When both the second library and the iCloud Photos Library are finished syncing they both with be the same and contain all of the Photos from the two Photos libraries.
   4. You can now delete your first/original Photos library (A).
   5. Disable the iCloud Photos Library in library (B).
   6. Cancel the iCloud Storage that was purchased to make the merge happen.

B - merge the two Photos libraries with the paid version of PowerPhotos.

1. Purchase PowerPhotos.
2. Set up to merge the smaller of the two libraries into the larger library.
3. Select to merge the originals.

NOTE: If you choose to merge the edited version only the edited version will be merged.  It's original version will not be copied over to the other library.  One an always reedit a photo but can't revert and merged, edited photo back to the original.  

Also using this method projects will not be carried over into the final library. So pick as the library to merge the one with the fewest projects that you can afford to not have in the final library

4. Once you've confirmed that you have all of your photos in the larger library you can delete the smaller one.

I would repeat the merging and transfer first the originals, and in the second run the edited versions.

At least I would keep a copy of the library with the originals on an archive drive. I frequently needed to revert the original when I no longer liked my edits.