The first thing you should do is to test your style in `compatibility
mode'. The only change you need to make in order to do this is,
possibly, to change the extension of the file to .cls
: you should
make this change only if your file was used as a main document style.
Now, without any other modifications, run LATEX2e on a document that
uses your file. This assumes that you have a suitable collection of
files that tests all the functionality provided by your style file.
(If you haven't, now is the time to make one!)
You now need to change the test document files so that they are LATEX2e documents: see LaTeX2e for Authors for details of how to do this and then try them again. You have now tried the test documents in both LATEX2e native mode and LATEX 2.09 compatibility mode.