How to modify the default style of part numbers in solidworks2007 drawings?

Are you talking about this style? For example, the default is underline, and I am a circle now. Is that what you mean? Click the part serial number, and then select a style in the design tree on the left. If you want to modify it later, you can modify the style in the menu bar Tools → Options → Document Properties → Remarks (click the plus sign) → Part Serial Number.

After modification, it must be saved as a drawing template. Drwdot format, and the template can be used to create new drawings in the future. Previous drawings can't automatically become the current style, so there are ways to modify them. You can find the template you just saved, copy it, and change the suffix to. Slddrt, open the engineering drawing that needs to be modified before, right-click in the blank space of the drawing, select "Properties" and then pop up the window.

Click "Browse" to find the file you copied and renamed, and select it.

Or you can directly click menu bar tools → options → document properties → comments (click the plus sign) → part serial number modification in the previous engineering drawing.