What kind of change do you want to make to the field width?
If you look at the forum post linked I linked to, Jeanie's post (bottom of the page) indicates that if you're only increasing the width, you can make the edit directly to the propdef table for those specific fields. Make sure to stop the server then start it again after you've saved your changes.
You shouldn't attempt any other changes directly to the table. This includes decreasing width, changing field name, changing type, etc. Increasing the width is the only safe change. Note that if they're willing to upgrade, it's probably better that they do so.
If you're looking to decrease the width of the fields, the customer is going to have to upgrade.