Recently I wanted to change the attribute type from 'Single line of text' to 'Multi line text' in CRM 2011. but we cannot do from CRM form. but there is a way to do it.
First you have to find the Single line attribute and multi line attribute from the "Attribute" table. and get the attributetypeId.
then simply update the Multi line attributetypeId with single line attributetypeId.
After that you have to Export the Entity and then Import it again to the CRM system.
Now open the Entity Form and remove the attribute and add it again to the form.
Save and Publish the Form.
That's it. now you will see the changes. btw, this is 'UNSUPPORTED' way.