how to convert and replace itunes m4a playlist with mp3 playlist

In Itunes, I want to convert a playlist of 200 songs from m4a to mp4 but don't want to have to struggle with creating the new playlist (ie, making a list of the songs and finding them in the library and dragging them into the new replacement playlist) or deleting the old m4a songs from the library (finding them and deleting them either by hunting them down or identifying the m4a duplicates and deleting them).

I know I can convert them en masse within itunes but then I've got to hunt them down in the library to recreate the new playlist and delete the old m4as.

I can't seem to come up with a smooth way of doing that. I thought I could drag them from itunes to my desktop, or even to a flash drive, but when I drag them back into itunes they don't come back in as mp3s but remain m4as (I did set the preferences import settings to mp3 so I don't know why that aspect doesn't work for me).

Any suggestions for how to do this efficiently would be greatly appreciated.