This is not a great way to build a reputation as a sporting player, but not illegal. The MTR 2.7 says
Registered decklists record the original composition of each deck and sideboard (if applicable). Once your decklist has been accepted by a Tournament Official it may not be altered.
That is to say - decklists are final when they are handed in to a judge, not when players take their seats for the player meeting, which most people are running at the same time as round one pairings these days. That said, I believe it is only *technically* legal, but far outside the spirit of decklists and competitive play.
This player is exploiting a loophole we opened by trying to save everyone time by going straight to round one pairings instead of having a separate player meeting. At an event the size of most PPTQs (most I've seen were ~25 players or fewer) we would have several options for closing the loophole, which might not work as well at larger events. The judge could start collecting decklists before posting round one pairings, or better yet, just speak with this player and educate them on the goal of decklists, why we are skipping a player meeting and going straight to round one pairings, and how they are not making themselves any friends with this behavior.