You can certainly give the 175cm Apache X's a whirl without too much fuss (unless you need to remount bindings, etc., just to try them out). Skiing them for a day will not reduce their resale value much, if at all. Their 175cm length might be plenty for you at this point. I just prefer to have a little more ski under me and I find my 184cm Mantras to be a delight. Some guys my size are skiing Mantras in the next shorter length (177cm) and seem happy as clams, which is great. But I would not want less edge under me, and I note occasionally people are selling close-to-new Mantras on eBay because they want the next size up for better floatation, etc. All of which indicates that how you plan to use the K2s will determine the optimal length for you. A 175cm is definitely going to be more nimble for you in bumps, trees and tight turns, with the inevitable tradeoff of a bit less edge-grip on hard snow, less stability at high speeds, and less buoyancy in fresh or ungroomed snow.
A big influence on new ski designs is their width. Offhand, I am not sure how narrow or wide the waist is in K2's Apache X. Lots of skis are getting much wider now, which gives them much better flotation for their length, yet they still have deep enough sidecuts for responsive turning and carving performance. My concern for you is that if the Apache X is fairly narrow-waisted and you are a strong skier packing 210 pounds, those 175's might sink like a stone in softer conditions. In that case, you are going to end up sticking to groomers, because your skis are simply unable to perform in satisfactory ways when you encounter powder and skied-up crud or you want to venture into the trees to find fresh snow. Either that, or you will need to ski VERY fast in ungroomed snow in order for your narrow-waisted skis to generate enough flotation to perform.
This was what I noticed when I transitioned from my relatively stiff, narrow-waisted, 190cm Volkl GS skis — which I loved — to the much wider, 184cm Mantras. Instinctively I found myself drawn to ungroomed snow, because those conditions are the "sweet spot" for Mantras. And I did not need to ski them nearly as fast or unweight as dramatically to get them on top of soft snow for turning. Given your skill level and experience, my guess is you will find yourself gravitating to situations where your skis perform their best. I ski differently on my Mantras because of where they are the most fun to ski.
Hope something in the assorted feedback you are getting helps. The main thing is that you get back out on the hill, enjoy yourself and build some fond memories sharing the companionship of other skiers! Equipment can enhance this, but is only a means to greater ends.