How to change default detail symbol from sheet name to sheet number?

Have read Steve Cocchi's  the posts on how to edit detail symbols but when one tries to edit the field that says 'sheet name' in the detail bubble, its properties are unavailable to be edited. Seems it is a system field. How does one change this field to be the Sheet Number instead of the Sheet Name?

The Sheet Names are too long and often overwrite the scale text.  We have always used sheet numbers in this detail bubble but need to find  way of changing the default to show as the Sheet Number instead of editing every detail bubble that is placed.