According to the linked statement from the UC Office of Scholarly Communications [1], "No matter what happens moving forward, UC scholars will still be able to use ScienceDirect to access most articles published prior to January 1, 2019 because UC has permanent access rights to them."
[1] https://osc.universityofcalifornia.edu/open-access-at-uc/pub...
Edit: dannykwells beat me to it, and in more detail.