How to convert text into long text?
Though the question is pretty old, I would like to add my suggestion to it. I don't think changing the field type directly in the database is a good idea. 


Create a new field (long text) with the necessary settings, and add it to the content type.
Then you can use the following script to copy all the old field values (text field) to new field (long text).



&lt;?php
// Assign value from old_field to new_field
$nodes = node_load_multiple(array(), array("type" =&gt; "your_type"));
foreach ($nodes as $node) {
  $node-&gt;field_new = $node-&gt;field_old;
  field_attach_update('node', $node);
}




Finally delete the old field and use the new field for further use.