if you want to do it to many colours at a time you can also set the stencil so that the colours you want to exchange are not selected (manipulatable), set the right-button colour to what you want them replaced with and press K (clear). I am pretty sure that it should work (no PM to test where I am atm)
I just checked and yes it works, but I was not saying many colors at a time but one by one,like: replace blue by dark blue, green by medium green, dark green by dark purple etc... so finally in this case, I think Miascugh 's method is the fastest.. you can easily set the palette by hue or brightness then editting the palette, copy a color over an unnecessary one, do this many times with different colors.. then delete duplicates and finally remap..
(I think adding a "Fusion with " button in the edit pallette would be cool

but may be it's just I who miss it , with my crappy methodology

)
thx again
