Two ways to pop the old knot
1. Snug the knot in a vise right at the edge of the handle, Crush, loosen turn 90° Crush repeat... Yank hard on the handle, If you are really lucky the knot will stay in the vise in one piece NEATLY... You will be holding a Handle that needs minor cleaning, to be ready for the new knot :)
2. Using a Utility Knife cut the bristles down to as close to the handle as possible making a complete mess of the shop and have bristles everywhere but in the trash can :p use a drill press and drill out the old knot...
Obviously try #1 first, only resort to #2 if you must, because it stinks and is messy (Pun intended)
TGN =
Fountain Pen parts - NIBS, - Wholesale Fountain Pen Ink - Golden Nib AZ they have the best selection of Knots to chose from that I have found to date
Hope that helps