2015 Retina Pro 2.9 ghz i5 to a 2012 Non Retina Pro 2.9 ghz i7. Very happy I sold the 2015 Retina, as I much prefer the Non Retina pro.
Good machine. If I remember correctly, those are easy to swap out the hard drive. If you don't have SSD now, you can get one and it wouldn't cost a fortune.