The series year typically changes with a design change or a signature change. When there is a new presidential administration with changes in the Secretary of the Treasury and/or United States Treasurer, the series will change. A letter, "A" was added to the 2017 series with a change in the ink or paper used. The BEP has been somewhat secretive about the latest changes. There are however, exceptions to this. Series 2006 $5 notes had a major design change without a series change. Both of these are series 2006 $5 notes.
Any idea when the first banknotes with Mrs Yellen's signature will be printed and then released? What has been the usual time frame for a series change after a new Secretary of the Treasury takes office?
If history is any indicator, Steven Mnuchin was sworn in February 2017 and as early as October 2017, series 2017 notes with his printed signature were being printed. If I had to guess, I'd say they will be series 2021 and released later this year.