I have noticed that changing a field length in Visual FoxPro does not change a table from a "FoxPro for DOS" type table to a "VFP" table. What does cause this change to occur?

I was "almost" horrified when I thought I had possibly changed original FoxPro 2.6 for DOS tables to VFP tables this moning. However, I am still able to open the tables (after only changing the field length of one field by one character length) in FoxPro for DOS, which tells me that I did not alter the table header and type.