How to transfer photo library from old computer...

How can I transfer my photo library on my old MacBook Pro (photos version1.5) to my brand new iMac (photos version 2)?

When I try to transfer the photos via file sharing - it wants me to update the photos app on my old MacBook (which I don't think I can), and my photo library disappears and I have to "restore" the photo library to see my photos again.

Help!