Converting SGML to XML

Hi All,
Even though this is my first post at the javaranch, I have been checking it out the last month or so and very impressed with this site. I help I will be able to contribute to the site eventually. Anyway here's my question:

I just start to learn XML recently for my new project. I need to define XML structure for a large document. I also have a SGML structure for the same document. Should I convert the SGML to XML or just start from scratch? Is there any tools to help convert SGML to XML? Thanks in advance.

There is an old, but good article "Converting an SGML DTD to XML" by Norman Walsh
Also "SGML Instance Conversion" may be useful. They mention James Clark's SX and SGMLNORM tool for SGML -> XML conversion.