The added info now gives a better picture. Thanx.
There may be a way to save the ivory ferrule that doesn't involve heat, or re-tenoning.
Assuming that the current ferrule is 1" long, chuck-up the shaft.
You want to remove 3/4" length from the shaft anyway so do that while the ferrule is still attached by boring that 3/4" amount of tenon out of the ferrule.
This leaves just 1/4" of tenon now holding the ferrule. Do you feel lucky?
The ferrule may break loose from the 1/4" tenon that is holding it.
Worst case scenario, the ferrule breaks. So what? You now have the shaft to the proper length for a new install of a new ferrule.
Might be worth a shot and may save you the cost of a new ivory ferrule.