Let’s say this takes place in season six, twilight sparkle has been having nightmares about that incident at the wedding rehearsal and wished it would never happen. So she comes up with a plan to travel back in time and tells her past self (While hiding her Alicorn wings from her) about The wedding rehearsal and the truth about the fake Cadance. Though she succeeded and the wedding rehearsal incident never happened, consequences followed soon after. So my question is what kind of consequences would there be?

Fluffle Puff and chrysalis conquer equestria

Depending on how the incident is avoided, you could have it so the Changelings are never exposed at the rehearsal and thus have much more time and planning for something worse since no one other than Twilight and or Cadance would even know/believe they exist.

Another way to do it is that even though the Changelings are exposed, without Cadance and Shining blasting them all away at the end, the disguised ones in Canterlot remain and thus are able to sabotage/spy on everyone there.

If Twilight warned her past self and convince her to gather clues instead of making a scene at the rehearsal. We wouldn't have one of the most polarizing episodes of the series.

THat being said, I think her changing the past would only delay Chrysalis' plans but if she brought the real Cadance as proof, then it would be prevented earlier.