Can you change your battlefield after a win/loss round in tournaments, so if I win the 1st of a best of 3 can I or my opponent change are battlefield

In a tournament, you use the same list throughout. Changes of any type are not permitted.

Cheers, going to my first one on Saturday so just checking

In all three games, I believe you and your opponent have to roll for initiative and pick a battlefield, so not all three games need to have the same battlefield (some of them can use your opponent's battlefield, some can use yours). But you have to bring the same battlefield to all the games.

So, if you bring Maz's Castle and your opponent brings Moisture Farm, maybe the first two games will use Moisture Farm but the third will use Maz's Castle. But there's no way you can trade out for Cargo Hold.

In best of three doesn't the loser of the previous game pick, thus only the first game is random?

4 hours ago, Amanal said:

In best of three doesn't the loser of the previous game pick, thus only the first game is random?

No its "random" every game, both players bring a battlefield to every game, at the start of every game you draw 5, mulligan, then roll out your characters winner decides which battlefield. Person who's battlefield you play goes first person who doesnt get to use their battle field gets 2 shields same thing no matter how many games are played. That's actually why whether or not your character combination is likely or unlikely to win the roll off is something that is in consideration with deck building as is whether you want your opponent to get your battlefield or not.

4 hours ago, Amanal said:

In best of three doesn't the loser of the previous game pick, thus only the first game is random?

There's some online tournaments who do it this way, but it's a custom rule.

You roll off to decide who chooses the battlefield in each Best of 3 game as well.

I just checked, and found that in the Tournament Guide, Page 5, point 6 contains: "In each game after the first of a best-of-three match, the player who lost the previous game chooses which battlefield to fight on."

It's pretty buried away, so very easy to miss.

Yeah... I was about to say, a best of x, it is rolled for the first game, then every one after the loser picks. Not sure why people thought this was a custom rule... But what others have said is also true, you have your deck list for the entire tournament, no swapping out.

12 hours ago, Plymouthdean86 said:

Can you change your battlefield after a win/loss round in tournaments, so if I win the 1st of a best of 3 can I or my opponent change are battlefield

If you mean change in deck - no.
If you mean change the choosen one from 2 opposing decks - sure you can, in first round its roll off to check who chooses BF, if it is Bo3 in subsequent rounds looser chooses one.