Iain, I did that last night - wasn’t too sure if it would work or not. I guessed on the .xml file, since it is the closest thing I could find to an OPML file. However I had over 4,000 ahead of me!
Oh crap - TheOldReader totally sucks! Now it tells me I have no subscriptions imported. Just this afternoon I had a little over 4,000 still ahead of me in their "queue". Note on my Home page says "Bad news, some queue positions got messed up again. We fixed the issue, and we hope this was the last time it happened."
So it appears they lost my import. I am done there. They don’t seem to have any clue as to what to do. I would question the reliability of such a program.