I don't know how much more value, if at all, a Series 70 Colt carries these days. It may be higher than the Series 80s due to the, umm, 'enhancements' made to the Series 80 pistols.
Any sight replacement will usually require both front and rear sights and the front sight would have to be removed and replaced with either a removable sight (dovetailed) or one that would have to be worked down to get your poa to poi. If I didn't want to bugger the slide and keep it original I would simply get an aftermarket slide (find them in the Shotgun News or from your dealer) and have that fit to the frame and use that while keeping the original in case you ever sell it. I did that with a Springfield, but just because the new slide came with good Novak type sights. HTH. Mikey.