Think Doyll may have got the two switched around, the Redux is the 4x Heatpipe version, and the normal U12S is the 5 Heatpipe version.
Between the U12S and U12A, there is a noticable improvement, again as others have said, through it's thicker (extended width) fins and also has more Heatpipes attached to each fin on the stack, meaning more surface area to let the heat go. As well as being given 2x A12x25 fans which help dissipate heat quicker. Both combined is where you see the comparison of the U12A and the D15 for performance.
But there are catches in each area; noise, temperatures, pricing, size, etc.
The D14, D15 and D15S may be able to soak a bit more heat via the two towers and at lower comparable fan speeds, may appear to be better than the U12A. Whilst the U12A may need to have somewhat higher fan speeds to match up, the A12x25 fans also produce better airflow and less noticable noise vs the fans on the D15. So if you're after the least amount of noise, this depends on other factors then, like how sealed is your case, how much heat the CPU pops off, and how good your hearing is (are you using headphones?) etc. Obviously, if the CPU and airflow in the case is insufficient, then the U12A will lose in certain situations where the temperatures of the CPU it is trying to keep cool, is simply beyond the capability of what it can handle compared to the D15 due to more area available on them.
So the variations keep going around between the two. But the key thing, which is what you've already mentioned already, is size matters. Since the D15 is going to be trouble for your expected new case, the primary trait that'll need to be considered first is size. Given that the D15 and D15S will extend too high height wise, they should automatically be ruled out. This then really leaves: U12S Chromax (Cosmetic + Price for buying again for no further performance improvement), U12 Redux (Weaker perfomance + lower price), U12A (Better perfomance + slightly higher price + Noctua colours).
Of these three, getting weaker performance for less price, but still needing to pay, seems silly. So this should rule out the Redux. Grabbing the Chromax may seem an OK deal, but you'd need to be looking inside your case more than your display really to find the colours of the cooler to be important enough to replace an existing identical cooler for a differently coloured one. Each to their own, but if you have the money and like looking inside your case more than your display, then by all means, go for it.
But since you're still going to need to change it later, then it seems kind of a waste of money more than anything. Then there's the U12A. Sure, it has better performance, but it's pricier than the other coolers. But if you're going to spend, you may as well get a bit more for the better performance one. Assuming it still meets the criteria you set out for it that is; It'll definitely trounce the D14, but the D15 and D15S will only get trounced, equaled or come close to depending on too many factors that vary (temps desired, noise, CPU power/temps, etc).
Since there's no direct competitor for the 120mm CPU air cooling tower that's competing at the same level, if you are going to reuse it anyway, may as well grab the U12A.
