I has been awhile since I put some MF's together, but the insert is about the same length as ones for aluminum. You can cut the shaft with a fine hacksaw blade (24-32 tpi).
Be carefull, the glass will split, or produce splinters. Smooth the end & inside with fine sandpaper.
The inserts were probably glue in with epoxy. They are almost impossible to remove without damaging the shaft.
If you just want to change the heads, use heat. Heat is an enemy of glass, because it will burn. Wrap a cold wet towel around the shaft as close to the head as possible. Use a propane or butane torch; a little heat goes a long way, then see if you can remove the head with pliers. Dip the shaft in water to cool.
If you cut the shaft, you can still remove them the heads this way. Epoxy takes more heat to break loose.
Cutting the shaft. Warning, make sure the cut is square and the insert fits snug, when you put it into your shaft. Do not force. Forcing will split the shaft.
I have some that I am going to shorten this way. I have thought about adding a short wood dowel into the shaft (loosely fit), and epoxy the dowel and insert in the same process. Reason being, the inserts are short, the glass is thin and occasionally shooting into wood will break or split the glass. What I am suggesting is the equivalent of a footed glass arrow.