replace mp3 with aiff

I built a large iTunes library from CD's and LP's starting back in 2006. In the early days, space was at a premium so I converted the aiff files I ripped to mp3 and copied the original aiff off to a hard drive for future use. My library is now a mixture of mp3 and more recently added aiff files. What I want to do now is re-import the higher quality aiff files, delete the MP3s while maintaining play counts, ratings and artwork. Information on how to do this is found with a search so I gave a sequence like this a go:

import the aiff into iTunes, quit iTunes, delete the mp3 files in Finder, manually move a copy of the AIF files to the location where the MP3 were, start iTunes and attempt to play the songs, iTunes will not find them and instead prompt for locating them, if iTunes shows both mp3 and aiff, delete the MP3.

This sequence did not work as planned. I don't think I wound up with double listings in iTunes. What I wound up with is the aiff files in iTunes with artwork and metadata with one problem. Itunes says the files are mp3. They most decidedly are not. The xml and or itl files do not appear to update or detect that the actual files are a different file type now. My hunch is that the advice on how to do this was good under an earlier version of OSX and or iTunes. I am running Yosemite and iTunes 12.0.1.26.