Interesting question from an event I ran tonight.

In the third round of an 8-man draft, the top 2 players know they will be playing against each other. If one of them decides to change up his deck to work better against his opponent is there any repercussions for that? Is this cheating? Does it matter whether they do it before vs. after announcing the pairings?

In limited tournaments that do not use decklists players may change the composition of their decks between rounds, unless the head judge announced before the tournament began that decks must be returned to their original composition before the beginning of each round.

So unless that was announced what the player did was legal.

Cris is spot on for the legality issue at Regular REL .

Be aware though that a community is influenced not only by what is legal but also by what the accepted norms are. In my experience, most Magic communities operate under the idea that players might tweak their deck along the way at Regular REL but that “pre-sideboarding” because you know what your opponent is playing is somewhere between bad form and complete jerkishness.

For example, Theros includes a cycle of color-hosers in that same color. If someone in my area runs into a blue-white mirror match where the opponent has main deck Gainsay and Glare of Heresy, that's going to seem awfully sketchy. Chances are, the community will shun, ostracize, and otherwise serve up steaming cups of Hater-ade unto that player. While a player's actions might have be technically legal, each community will decide for itself what it determines as acceptable.

Note that as judges, we are often actively or by our own example looked to to set standards. The standards we set can have positive and negative impacts on our communities large and small.