They're swappable, except for the spacebar, on which the Leopold has different stabilizer spacings.
All the Leo and Filco keys are ABS, and the Filco blanks are uncoated (the printed Filcos have a coating on the key) and thus, the texture is about the same, I've heard. The Leopold still has thicker keys, which make keypresses feel a bit better.
On the other hand, Filcos have the standard spacebar if you want to buy a PBT set of keys, and also better stabilizers, making the larger keys (Shift, enter, spacebar) feel better.