Howto downgrade LyX 1.6.1 documents to LyX 1.5.5 documents?

[Lance Simmons]

I've switched from Debian unstable to the new stable release, but now now I
have a few dozen LyX 1.6.1 documents that LyX 1.5.5 says it cannot convert.

How can I convert the 1.6.1 documents to the older 1.5.5 format?

-- 
Lance Simmons