There's a high-profile cycle of color hosers in the new set, Chandra's Defeat and friends.
Just wondering, are we allowed to modify continuous construction to “you can't change your deck anymore once you know your pairing”?

I see more people at regular that only take out their sideboard cards when they sit down for their next match than people that plan on putting specific cards in. Would you want to enforce this no construction thing on these players as well and get them stuck with their sideboard cards from a previous match? Or would you need another exception for that?

Let's have a look at the MTR!

Originally posted by MTR 7.3: Continuous Construction:

Players participating in Limited tournaments that do not use decklists may freely change the composition of their decks between matches by exchanging cards from their deck for cards in their sideboard without being required to return their deck to its original composition before their next match.

One could debate when a matchs starts, but I believe that it is common practice to interpret “pairings have been posted but the round hasn't started yet” as “between matches”. So no, the MTR currently doesn't allow this… and Quinten gives a very good reason as to why not.

Originally posted by Dustin De Leeuw:

Let's have a look at the MTR!

Unless I'm mistaken, the original question wasn't about what current policy is, but about whether we can slightly modify it to better fit the intent.

There have often been hosers in the format while Continuous Construction was allowed. Why should it be different this time?

Remember that Continuous Construction doesn't apply if decklists are in use - in other words, if there is a valuable prize on the line then it won't apply as we'll be at Comp REL.

At Regular REL, we have to weigh the confusion and inconvenience to less dedicated players against the feel-bads those players get because their opponent will, very occasionally, have the perfect hoser, learn from their friends that it's relevant, side it in, draw it, and blow them out… seems like it won't actually happen all that often.

Although it is somewhat unfair to the opponent who is being hosed, it is still entirely legal.

Can't really try to enforce a rule that doesn't exist.

It is also entirely legal (and karmicly satisfying) for the opponent to
change decks or colors entirely, thereby sticking the player with a dead
card in their deck. I do this quite frequently in Continuous Construction
tournaments, particularly Sealed deck, particularly pre-releases. Granted,
it's unlikely they'll know this plan will have the benefit of messing with
the opponent's pre-boarding, but it could still come up.

The serious question, with no joke side at all, that this brings to me is,
where do we draw the line? As I said, I quite frequently try two or three
different decks at events like Prereleases. Sometimes, the difference
isn't so much “is this color better?” but rather “This deck looks like it's
better vs. a slower deck” or something similar. Is that pre-boarding?
What if I choose my deck without knowing what my opponent is playing? What
if you just kept an eye on the top tables and concluded there was an 80%
chance that X's Defeat would be useful and you decided that was good
enough? What if you just forgot to board it out? To me, this change
complicates a relatively straightforward policy for little gain.
Prereleases aren't supposed to have very high stakes anyway, so if someone
is eking out a slight edge this way, are we really worried about that?

I would tell the player that although pre-boarding is technically allowed, it's not very sportsmanlike and I expect better from them because a Regular event shouldn't be just about winning.

Originally posted by Justin Miyashiro:

What if I choose my deck without knowing what my opponent is playing?

Then it's not unsportsmanlike, just smart (if you guessed right.)

In the end, I think, this comes down to enforcability more than anything else. Players are currently allowed to “pre-sideboard”, and I think most of us would prefer that they didn't. However, if we cannot enforce a rule banning this practice, then everything else is moot. If you see someone doing it, by all means, have a quiet word about sportsmanship, but I don't think you can do anything else.