how to copy data from one custom field to another (text to textarea)
To find the correct table(s), you need to know the id of the custom field as well as the id of the custom group it's in.


Go to Administer menu » Customize Data and Screens » Custom Fields
Click View and Edit Custom Fields next to the relevant custom field group.
Look in the URL - it should read something like:

http://mysite.org/civicrm/admin/custom/group/field?reset=1&amp;action=browse&amp;gid=4

Look for the number after gid= (in the example above, "4").
In phpMyAdmin, look for the tables starting with civicrm_value.  Find the one that ends with the gid you identified above.
You should be able to find your custom field data in there.  If your fields are very similarly named, you might also need to know the custom field ID.  In that case, click Edit Field and note the id in the URL.  The custom field you're looking for ends with that number.