I'm tentative about the whole groom swap thing, it worked at Karan and Monisha's wedding because Preeta and Monisha are of the same height and built, but if a groom swap were to happen between Prithvi and Karan it would be too obvious, there is an obvious height difference and physically they are very differently built people as well. It sounds odd even while I'm writing it now, but Rishabh and Prithvi have a similar structure as opposed to Karan and Prithvi.
Then the only thing that can happen is, Preeta running away from her own mandap for Karan (shaadi won’t be stopped because of Prithvi being exposed).Which is again very unlikely because she wouldn’t want to hurt her family, etc, etc. Let’s see! The writers have always come up with some illogical thing.