As I understand your needs, you need to choose the function name that you want to use. Usually you need to use rules to do that.
For example if I define two simple functions
f[x_] := Sin[x];
g[x_] := Cos[x];
and later need to use some UNDEFINED function, say h[x] which will be set to be f[x] or g[x] then
h[x]/.h->f will change the head (the function name) from h to f and you will finally get Sin[x].
Ofcourse you may change the x itself in a similar way.