The message indicates that the import filter detected the file as using the original LaTeX format, known as LaTeX 2.09, rather than the current LaTeX2e format (now just known as LaTeX).  LaTeX2e was introduced in 1994.

LaTeX 2.09 documents are characterized by using the \documentstyle statement to specify the base typesetting specification.  LaTeX2e uses \documentclass for this purpose.

A quick hack to convert from LaTeX 2.09 to LaTeX2e is to use a text editor on the .tex file to change \documentstyle to \documentclass.  This is not completely reliable as there are other aspects of a document that are different between the two formats.

A more reliable conversion method using SW 5.5 is to start a new document (all the Version 5.5 shells use LaTeX2e), and then either use File, Import Contents to select the previous document, or use copy/paste to transfer text from the old document into the new document.

The modified or saved .tex file can then be imported into Version 6.