converting DocBook SGML to XML?

It appears that WYSIWYG editors (and perhaps other tools) are
more generally available for DocBook XML than for DocBook SGML.
So, this makes me wonder:

   *  Is there a reasonably reliable tool chain for converting
      DocBook SGML to XML?

   *  If not, how challenging a project is this likely to be?

Given that this is in the context of a one-time conversion, a
tool chain that did a reasonably clean "first cut" would still
be of interest.  My assumption is that human editors could fix
occasional botches by hand.

-r
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