title: Upgrade Mongo DB Error - "Unsupported WiredTiger file version"
body: <p>I'm having the exact same issue described <a href="https://jira.mongodb.org/browse/SERVER-35271" rel="nofollow noreferrer">here</a>, but with diferent versions:</p>
  
  <ol>
  <li>I have a 3.4.7 MongoDB database with data <strong>I can't lose</strong>.</li>
  <li>I upgraded to 3.6.9 </li>
  <li>Set FCV to "3.6"</li>
  <li>I upgraded to 4.0.20</li>
  <li>When I try to start server I got this error</li>
  </ol>
  
  <p><code>2020-01-30T14:17:19.932-0500 E STORAGE  [initandlisten] WiredTiger error (-31802) [1580411839:932626][32464:0x7f126518da40], connection: __log_open_verify, 1028: Version incompatibility detected: unsupported WiredTiger file version: this build requires a maximum version of 2, and the file is version 3: WT_ERROR: non-specific WiredTiger error</code></p>
  
  <p>If I delete the data directory and start MongoDB 4.0.10 it works, but I can't add it to the 3.6 replicaSet I have with the other nodes to sync and recover data. So if I follow this process in all nodes I will finish losing all my data.</p>
  
  <p>What else can I do?</p>