Well the replacement of WMAs with MP3s is working, at least in some cases. I use the Groove app in Windows 10 (pre-Anniversary release, I believe). I'm looking through my purchased music folders one at a time (in my Zune folder structure, purchased music is in folders by artists\albums directly under the Zune folder). When I see WMA files, I right click on the album in Groove and click download; sometimes it starts downloading the track(s), sometimes it gives me an error saying that the album is no longer available. The tracks start getting download in Zune\Music Cache, then get transferred to \Zune\Purchased. I'm moving the MP3 files into my regular folder structure mentioned above.
So far it's worked with two Miles Davis albums with very long tracks.
It doesn't work for the Sly & The Family Stone at Woodstock release. When I purchased this, I was trying to use my 10 tracks per month credits, but it instead purchased the whole album, a combination of the Woodstock tracks and the album Stand! I had to pay extra for that, and now I can't convert my WMA tracks to MP3s because that configuration of the album no longer exists in the Marketplace. I'll have to write customer support to try to recover them. I'm afraid the WMAs won't be playable after this change.