A convert rule is what convert-ly uses to update old .ly files. All
the rules are contained in python/convertrules.py.
A full LSR update (running makelsr.py on the docs tarball from LSR)
calls convert-ly on all the snippets.
I have no problems adding this to D/S/N and then running makelsr as I would
for any 'new' snippet if that is ok?
We only move an LSR docs snippet to snippets/new if it can't be
updated using convert-ly (this usually means anything more complicated
than a simple search-and-replace).