You cannot switch those two without switching something else (most probably two other similar corner-centers, on any of the other sides). Once you realize this, you can work out a commutator (or two commutators right after each other) that can do the switch. If you need to swap A-B and C-D pieces, use these commutator cycles:
A->B->D(->A)
C->D->A(->C)
These two overall will swap A with B, and C with D.