precisely... he's already the duke of that region. he just want to flip the region where he is lord.
the fact that he wants someone else as duke too, based on his old region, is a separate matter.
now, if the newly captured region does not belong to his duchy, then he's a bit buggered. because that'll probably involve some other duke appointing someone else as lord, for that lord to have him as duke, then step down... then he flips region as above, and appoint the lord in old region, then create duchy.
... in short, forget it really.
unless you've already coded the bit about 1 duke giving 1 region away to another duke?