Take the sickle out and lay the bar on edge on an anvil, sections hanging over the side and strike them with a heavy hammer. It'll shear the old rivets off and you can knock them out of the bar w/ a punch. Those tools don't work too good to remove rivets.
I'd go with bolts when replacing sections. If they hit the hold down clips, change them to the newer style. I've never had a problem switching from rivets to bolts though. They sell a tool that presses the bolts in the bar and then it's really simple to fix when a section breaks in the field.
One other note- The hold down clips need to be snug on the top of the sections and the sections also need to ride on the bottom of the guard or the ledger plates.
Where in Appleton you at? Our farm is on the South side of Kaukauna, on 55 near KK.
Donovan from Wisconsin