If that is true it also has to be happening with the potassium you get from food, your body is not just going to say ooh here is some potassium from a supplement, lets turn that into calcium but leave the potassium from a banana as potassium. That is completely illogical.
I actually hope it is true that our bodies are converting potassium to calcium as I have taken so much potassium over the last 3 months that I am bound to be out of 4 lows.