If I was in your position, I would study for the normal LSAT right now, then switch over to preparing for the Flex if/when they announce it. I have been studying for the June exam; originally preparing as if it was a normal exam, then switched once they announced it was a Flex exam. I do think it helped my mental stamina when doing the timed prep tests for a normal exam because going from 4 sections to 3 sections is noticeably different after I finish. Plus, if you prepare now by only doing 3 sections, then come test day you have to go in for the standard exam, it is actually 5 sections (1 section not timed). However, Flex is only 3 sections in total.

Word.