First, make sure you have a complete back-up. IIRC when I installed, the 2011 version stayed in place, so I at one point had both 2011 and 2016 installed, though now only 2016. When I deleted the 2011 version, it only deletes the program files. Your data files stay in place. On a somewhat related note, you may have something loading in your OS that is causing 2011 not to work. Check Login Items for a start, and/or Etre Check (free) to see if there are extensions/etc that may be loading and causing you troubles. This is pretty common for people (like me!) who have had their Macs for a long time, updating application software, etc.