I'm picking door number 3 - "Take it all the way down to bare concrete." If you're going to use a system without a primer, it's possible there's nothing to gain by removing all the existing epoxy. But if you're using a complete system, including a quality epoxy primer, then you'll want to take full advantage of the primer and allow it to adhere directly to concrete.
Not a complicated formula here - "A chain is only as strong as its weakest link." When you review all the posts here and elsewhere it becomes clear that the main point of coating failures is inevitably the intersection where that first layer of epoxy meets the concrete. That's where we find hot tire lift, peeling, etc. Occasionally there's a case of separation between coating layers, but that's unusual and often traceable to incompatible material or application error.
A quality primer doesn't just adhere to the substrate mechanically, it penetrates the concrete and also bonds chemically, but that requires that it comes in contact with the concrete.
So while a typical pigmented epoxy base or top coat may adhere equally well or poorly to both concrete and an existing coating, and with the pre-existing coating your system integrity will depend on how well that earlier coat stays stuck.
Whereas a epoxy primer should adhere better to the bare concrete than the pre-existing coating.
So it also depends a bit on the specifics of the existing coatings and the specifics of the replacement system. But regardless of those details a good rule of thumb here is -don't take short cuts.
