our systems engineer took it upon himself to upgrade to 2018 without telling or asking anyone (we work at a small company). now all of his designs are saved in 2018, but us with 2017 can't open his files unless they are converted to .STEP (losing all feature tree history... you all know the score).
i just briefly looked at his system in the "save as..." dialog but couldn't find any options to save as a legacy type solidworks file. of course, he said that 2018 files were backwards compatible with 2017 (they aren't). and his designs are for injection molded plastic, so the idea of recreating the parts is its own headache.
and i feel like i shouldn't have to answer the question "why don't you upgrade?" both him and our electrical engineer's systems were down for a week and a half after they upgraded (systems engineer upgraded to 2018, which required out EE to upgrade to PCB '18, which was fine until he was tasked with doing all this plastic concept design work...)