They have to do it manually, and you'll simply need to prove the feed tot eh old site was you, no idea how it works though. Could set up a new feed account, but then you lose whatever readers you've got, better to get them to change it.

It's in the support or syndication section of the LJ FAQ, I read it a few months back but have never done it; g'luck (and let me know, I may need to do it soon as well).