ESL tournaments to transition to CS2; IEM Sydney first big LAN
Online leagues like ESL Impact and ECL S46 will be making the switch when they resume in the first week of October.

ESL has announced that its tournaments will transition to Counter-Strike's new iteration as soon as October 2 when ESL ANZ Champs 2023 Season 2 kicks off. Events will use the MR12 format to keep action in line with what players experience in-game.
Online events like ESL Challenger Jönköping qualifiers (October 3), ESL Impact League Season 4 (October 4), and ESL Challenger League Season 46 (October 5) will soon follow. In the league formats, deciders will be played as tiebreakers after the switch to MR12 pushed ESL away from round difference.
The exception to this is ESL Pro League and ESEA S46 at Advanced and below, which will remain on CS:GO.

This means that IEM Sydney, starting on October 16th, will be the first Big Event to be played on CS2. The tournament organizer admitted it would be more "complex" to run a big LAN event on CS2, but are confident of adjusting in time.
After Sydney, all competitions will transition to CS2, including qualifiers and the full 2024 calendar.
Justification for the rapid transition comes from an expectation of "very limited" practice opportunities for CS:GO, something that Vinicius "VINI" Figueiredo has commented on as a challenge as his side prepares to tackle BLAST Fall Showdown next week on the old iteration.

The first ESL tournaments on CS2 will be the ESEA and ESL Impact Cash Cups, weekend events available on FACEIT.