If you read the rules before registering for the tournament, there is a clause that says: 3. The organizers may make changes to the rules and regulations during the tournament if necessary. Changes regarding the chess timer were made after a vote was held on it, and it was approved for all tournaments on the portal. Read the rules first, then ask questions. This message is addressed to everyone who has any questions; further questions will be discussed in ICQ.
Sincerely.

If I understand H2SO4 correctly, the question wasn't about what the organizers can or cannot do, but rather about what specifically caused these changes.

All I could find in the three topics below concerned changing the values of the chess timer, and nowhere does it say that the chess timer becomes the main one, nor is such a point included in the votes. In the last topic, Pit for LM in post 4 states that the choice of timer remains at the discretion of the tournament organizers.
Please correct me if I am wrong.
/topic/11010/
/topic/11031/
/topic/11076/