Probably fifteen years. Sector editor may work?
Might a "sector editor" work?
Possibly, probably not.
But if you knew how to use that, you would have just done it and see what happens...
Maybe try a different floppy drive, before blindly poking around with a hex editor on the actual data.
Additionally, you don't mess with the data on these floppies. You instead do a sector by sector clone to some other media, and poke around in there.
What could possibly be on these 15 year old floppies that is "important"?