I would be careful trying to use the same power system on two different sized airplanes. It's a great money saving suggestion, but it might turn out you saved nothing in the end. Imagine trying to needle back (detune) your .46 on a ringmaster.
If you want to run the AXI 2826/10 on a 3S battery you will have to turn a large diameter prop with very high pitch at a slow rpm. For a more successful setup what should be done is use a motor with less winds (higher Kv) like the 2826/08. Changing motors (winds) defeats the purpose and your back where you started. At that point you might as well go down a motor size or two and save money and weight. Trying to use the same motor on 3S and 4S batteries is a tough job (at least in C/L).
If you are going to build two airplanes using the same power system, (say a Ringmaster and P-40 for arguments sake) the only savings would be on the purchase of one motor ($81.50 for the 2814/12 and $85.60 for the 2820/10). I don't think the money saved is worth the risk of having a poorly flying Ringmaster and good flying P-40 or vise versa. Why not spend the extra $80 and have two well setup power systems. You already bought the 3S and 4S battery packs, ESC and timer.
Of course if you plan on buying both power systems, or would like to experiment, try the larger motor on the Ringmaster with the smaller battery and let us know how it works. It may be an unusual setup, but it may work OK.
Rudy, are you are talking about Will Moore being the first to use the AXI 2826/10? I may be wrong (correct me if I am), but I believe Walt Brownell was the first to use it. He flew this setup at the 2005 NATS.
Again good luck and let us know what you decide on and how it works.