I yanked the plastic liner out of my BK9 sheath with a large needle nose. After forming with a heat gun I expected to have to put some more epoxy like shoe goo to keep the liner in, but didn't need to as it retained the knife well after reinsertion.
BTW Sheath Tricks: Drill a drain hole in the bottom of the plastic liner. Wrap the liner with paracord. Duck tape a fire steel blank rod to the liner. Use spray paint to camouflage as black is a poor camo color, it'll wear off around the edges where it rubs but works okay. Use molle straps or alice clips around a belt loop so the sheath doesn't wander as much. A small diameter piece of bungee cord or surgical tubing makes a good leg strap.