Not sure if you considered it but unless you want the challenge of de-fretting, filling slots, setting up, etc. why not buy a genuine replacement neck for a couple hundred bucks? That way, if you don't love it then it's just a simple reversion back to what you have and something to sell to recoup some cash. A low risk way to try what you're thinking. I believe the ReBop necks are available in all three flavors, fretted, lined and unlined fretless. I was mulling this option for my current project.
Let me throw this in as well (and could be way off course in my thinking). Do you currently have enough range left to lower your bridge (or have a shim to remove under the bridge) that will accommodate your preference of action? Would a new fretless neck have a slightly thicker fingerboard to make up for some of the loss in the height of the frets on a fretted neck? If you can't lower the action enough for your liking and the saddles are max low then you are forced into other means of lowering like filing the saddle slots. Once you're there there's no going back without some expensive replacement parts. Does this make sense?