I recently started CS50, but I'll have to finish it up in 2015. Do I need to un-enroll to start again?
CS50x 2014 ended on 31 December 2014. Didn't finish in time? Not to worry! CS50x 2015 (aka CS50x3) started on 1 January 2015. To enroll (for free) in CS50x 2015, visit cs50.edx.org. You may re-submit problem sets that you completed in 2014 for credit in 2015. However, know that some of 2015's problem sets are different. Here's what's new in 2015.
It seems, however that ther is no explicit "need" to unenroll from 2014's cource, it just will become obsolete after a while.
So on my opinion, basically, save your work from 2014, enroll into CS50 2015, install the new tools (the new appliance, etc.) and after migrating everything (and resubmitting unchanged psets to 2015), unenroll from 2014.