Q:  I edited the bill format (Setup > Bill Formats) but the bill didn't change.  Why not?

A:  Each matter specifies which bill format it will use.  Most likely the bill format you edited is not the bill format that the matter uses.

Edit the matter.  On the Billing-1 tab, the Bill Format box shows the number of the bill format for this matter.  That's the bill format you must edit to change this matter's bills.