Anytime you could sideboard you can change basic lands as you please. You effectively have access to any amount of basic land, of any types you'd like, in your sideboard at all times.

In an event with decklist registration, you're required to return your deck to its original registered configuration before game 1 of each match. You'd be free to fiddle with your ratio before games 2 and 3 of each match but game 1 you're always playing exactly what you registered.

In an event without decklist registration, you're free to sideboard and tweak as you please, with no requirement to return to initial composition at the start of a match.

7.2 Sideboard Use

Any drafted or opened cards not used in a player’s Limited deck function as his or her sideboard.
Players may request additional basic land cards for their sideboard. There are no restrictions on the number of
cards a player may exchange this way as long as the main deck is legal afterwards. Cards do not need to be
exchanged on a one-for-one basis.

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. The Head Judge or Tournament Organizer
must inform players if this option is not being used prior to the start of deckbuilding. This option is not available
at Competitive or Professional REL tournaments.

I believe in the events where you don't register decklists for draft/sealed you are not allowed to sideboard between match postings and the first match of the round. To avoid situations where you scouted the other decks and pre-sideboard for the opponents deck. At least that is how we always played it.

No, you can change your deck at any time you like between matches--you could even sit down in front of your opponent for the next round and change your deck right in front of them, explaining to them point-blank how the changes you're making will improve your matchup. It's major angle-shooting, and if you do it you're probably taking things way too seriously and the people around you are going to think you're a jerk, but it's legal.

No, you can change your deck at any time you like between matches--you could even sit down in front of your opponent for the next round and change your deck right in front of them, explaining to them point-blank how the changes you're making will improve your matchup. It's major angle-shooting, and if you do it you're probably taking things way too seriously and the people around you are going to think you're a jerk, but it's legal.