Thanks, and sorry for the delayed response.
Our intention is that the change would not affect existing students at all. Existing students are still in the original 3-term, 9-month structure.
Our student experience team (selfdrivingcar-support@udacity.com) can provide answers to more specific questions.
Update: We kept the 3-term structure of the program open for almost a full additional year, so that any student who started the 3-term structure had ample opportunity to graduate.
We have now closed the final cohort for the 3-term structure. This means that any student who started the 3-term structure and who wants to finish will have to finish in the 2-term structure.
The good news is that almost all the content that was available in the 3-term structure is also available in the 2-term structure. However, in the 2-term structure, many of the previously mandatory projects are now listed as optional.