My heads are moulded with the complete neck, for anatomical realism when wearing open-necked shirts etc.,
so they need a different approach from the conventional vinyl head.
Movement is very restricted, since they don't have the obtrusive joint on the neck.
Check the fit of any headgear before painting : you may have to clean up the mouldline a little , and perhaps reduce the hair to get the best fit.
Remove the vinyl head by heating it with a hairdryer or hot water, then pulling or levering it off with a screwdriver.
To fit the new head : there are two approaches :~
First, if your figure's uniform has a high collar,
you can of course carve away the sternum and shorten the base of the neck :
the resin is easy to carve with a heavy sharp knife ; you can also use a router on a Dremel drill.
The neckpost can then be routed down until the head fits over it : you may need to remove quite a lot of material , and also open out the top of the chest.
It's better to remove material from the neckpost rather than opening the inside of the neck too much : remember the resin is already fairly thin, and you don't want to crack it.
The second approach , which I prefer, is to remove the neckpost entirely, open out the top of the chest with a router , then fit a new neckpost from Sculpey or epoxy putty :
This is essential if you want an open neck. Of course , there is very little rotation, but that's the penalty of realism.