Why would you want to do this? Not sure but a sample here:
[as]var _x = 100;
function a() : void { trace("\t\tCalled from A = _x" + _x + "\n"); }
var b = function (): void { trace("\t\tCalled from B \n"); }
trace("Do a regular B call\n"); b();
trace("Convert B to A\n"); b = a;
trace("B Converted\n"); b(); trace("Do a regular A call\n"); a();[/as]
Result
[as]Do a regular B call
Called from B
Convert B to A
B Converted
Called from A = _x100
Do a regular A call
Called from A = _x100[/as]