Not sure what kind of joint that fig has (disc/peg?) but I've done this plenty of times. Bear with this if it's overkill, trying to tailor responses as "how to" material.
Conical Dremel grinder bit
Conical Dremel sander bit
Garment Steamer or Tea Kettle
Sculpting compound or clay
Revoltech joint or fodder peg
***DISCLAIMER--IF NOT DONE CAREFULLY IT IS POSSIBLE TO BURN ONESELF SEVERELY WHEN WORKING WITH BOILING WATER***
Kids, have your parent or guardian supervise and always remember--safety first!
1. Remove the head from the peg via "heat and pop" method with steam from garment steamer or kettle.
2. Remove the disc itself also using heat and pop. If you break the neck housing at all, try to make a clean break so you can just glue it back on once you have the disc out.
3. Starting on the slow setting, Dremel out the neck housing from the very top of the neck (as if you were looking from on top of the head) using your conical grinder bit. Adjust speed as you bore into the housing.
4. Switch to the sander bit and insert the dremel bit about 2/3 of the way into the neck housing. Dremel evenly on high speed.
5. By now you should have a nice hole in which to insert your previously mixed Apoxie, Green Stuff, Sculpy or whatever. Fill in the neck hole generously, removing any excess.
6. Follow Automatauntaun's guides on seating joints into uncured sculpt--if you have a regular peg, position as desired and just leave it in the housing while the compound dries. You will not be removing it with this type of fixture. If you have a Revoltech, insert it until snug and then carefully, slowly remove.
7. Let compound cure or boil water and immerse the upper torso of the figure into the pot to harden clay.
8. Repeat steps 3-7 as/if needed with the head you want to use. If using a Revoltech joint, it may be necessary after this to drill deeper into the head using a 1/16" drill bit for best joint fit.
9. You should now have a super poseable head/neck joint with capacity for swappable heads! Sand and paint your modified necks as needed. You can use this procedure to create a variety of combinations for heads and bodies that aren't compatible by design!