Hi tc..
Steve is correct,, a heat gun is the way to go....
Now before you go and play with it,,let me ask you a few questions ???
How much bigger is the new handle ??
If it is quite a bit larger and different in shape,, you may have a problem...
On the other hand,, if it is only a minor change, you could get off easy and stretch it in...
The best way to do this is heat up only the mouth of the sheath...
don't go towards the blade at all..
Heat very carefully and use a shield like a towel or something where you want the heat to stop..You can even wet it a little as an added precaution...Wrap the towel, slightly tight,don't over do it.
Now heat slowly until the plastic starts to deform. Slide the knife in and leave it..
You can mold it with your hand a little if needed to shape or stretch it around the handle..
Once its cool, do your final fit and finish and adjust as needed..Trim and sand if needed and your all set...
Just be careful you don't overheat, and keep the gun moving so you don't get shiny spots..
good luck...
ttyle
Eric...