If dems win senate, what day does the majority leader change? Immediately?

On Inauguration Day.

In a 50-50 split, the VP is the tie-breaking vote, and therefore determines the majority.  Which means that it's Pence until Harris is sworn in.  And McConnell is majority leader until the same date.  And likely Schumer afterward.

If it had been 49-49 pending the two runoff elections, winning both would have made it an outright Democratic majority.  Which would go into effect as soon as the two Senators were sworn in.  

If the Democrats had won the Senate outright on November 3, the majority would have changed this past Sunday, when the new Congress was sworn in.

Actually, even with both seats, Ds have an effective Majority - but not an actual one.

Effective because VP cast vote to break any tie - but it would be an actual 50/50 split.

I am looking forward to a check for $2,000 and legal marijuana. 

Everything, get ready for crying from SEA TO SHINING SEA.