That's not what Jeff Merkley said, Bobert. According to the interview I heard with him, if the Senate does not declare itself "a continuing body" on Jan. 5 and motions are introduced to amend the Senate rules, those motions can be debated for some time - no real time limit - before being voted on, and the vote on them requires only 50% "Yea" votes (plus the VP), even if the actual vote doesn't happen for a while.
In other words, proposed rule changes must be made on the opening day of the new session, but the vote does not have to happen that day, if Senators are still debating the proposals.
Once the rules for the new session have been established - either to amend the old rules or continue with them unchanged - then it takes a 2/3 vote to change them.