Not necessarily; the ladder reset just resets your overall games played (win/loss ratio) and that's about it.
Your MMR is not affected, so essentially you'll have one placement match but will still have your existing MMR; so you'll most likely be facing an opponent similar to the ones you're facing at the moment. So ultimately it's not going to result in a promotion or demotion, in reality.
I don't think you read the entire notes:
"Leagues will now be locked during the final weeks of the season. As a result players will not accumulate additional bonus points and will not be promoted or demoted out of their current division during this period.
This will allow players to focus on moving up in their current league without being promoted during the final weeks of a season, and having to fight their way up from the bottom ranks of their new higher league."
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