"their offer to substitute non-protected files"
Ah, wish I'd known about that while it was available. Anyway, thanks for your response. Here's what I've found out:
1) Re-downloading from my Purchased list just gets me M4Ps.
2) iTunes warns me that I can only burn a Playlist with protected songs to disc up to 7 times.
3) Burning to a DVD-R as a Data CD gets me M4Ps. These get re-imported as M4Ps.
4) Burning to a CD-R as an Audio disc (NOT MP3 format) gets me CDAs. These get re-imported as MP3s which are 50% larger than the corresponding M4Ps, so hopefully the quality is decent.
5) However, I can only fit about 17 songs on a 700MB CD-R (and I have around 120 M4Ps to convert). I only had one blank CD-R, but I was able to successfully get 17 songs converted. So I've ordered a pack of blank CD-Rs to enable me to do the rest.
Thank you SO MUCH for your help. I would have been extremely unhappy if I hadn't been able to find a way to do this without paying for the songs all over again.