I, for one, am big on being 1 step ahead of the 'game'. Depending on who the mfgr. is, you could VOID your warranty even if it's installed by a professional.
You see, most mfgrs. require a smooth, flat surface. If you don't really have that, the Installer VOIDS the Warranty. Yes, it's done all the time, but w/o seeing your home, knowing where you live, and a host of other variables, in a year or 2, maybe your original siding starts coming loose, or maybe it's rotting, or maybe the nails start 'popping out' from the siding, and so on. If you have tongue and groove 'anything' for siding now and it's smooth and flat, then I don't see a problem. But let me tell you from experience, a LOT of times you won't see the 'imperfections' of the old siding until the NEW siding is installed.
Again, w/o being able to do an on-site evaluation, get 2 or 3 estimates and ask the Pros why you should remove the existing siding, and ask them why not (depending on what they say they'll do.) BUT, I DO suggest that before the Installer does ANYTHING, ask for a copy of the Warranty and then contact the mfgr. yourself if you have any questions. Be sure to get names and phone #s of those folks you get 'info' from.