title: How do I add roles to existing user in MongoDB 2.4
body: <p>I've spent the obligatory hour trying to dig through MongoDB's online documentation, which is pretty shoddy for version 2.4, when user admin commands were in flux.</p>
  
  <p>I've figured out that in mongoDB 2.4.11 </p>
  
  <pre><code>db.addUser( { user: "...", pwd: "...", roles: [ ... ] } ) 
  </code></pre>
  
  <p>works. But I can't find any example of a command to update a user's roles, and I get this error message when I try to just overwrite the existing user:</p>
  
  <pre><code> User already exists with that username/userSource combination at src/mongo/shell/db.js:125
  </code></pre>
  
  <p>So I'm stuck.</p>
  
  <p>(and as bonus, how do I remove a role from an existing user in 2.4?)</p>
  
  <p>When I try </p>
  
  <pre><code>db.grantRolesToUser( "myself", [  {role: "clusterAdmin"} ] )
  </code></pre>
  
  <p>as documented in v2.6, I get: </p>
  
  <pre><code>TypeError: Property 'grantRolesToUser' of object admin is not a function
  </code></pre>
  
  <p>so in 2.4 not working.</p>