How to rename the automatically generated task form in SharePoint Designer workflow

In SharePoint Designer click on All Files in the Site Objects. In the All Files tab navigate to Workflows and click on your workflow. You should see the xoml file, the rules file, the xsn forms and a wfconfig xml file.

To rename the xsn form, update the wfconfig xml file. Replace the old name with the new one. That xml file contains the content type related to that task, so you can also change the contenttype name as the 'WorkflowForm' name. After saving the wfconfig xml file you need to restart SharePoint designer and you should see your form with the new name.

You need to be the site administrator in order to see the All Files link un Site Objects. 

Also it looks like that Lookup type does not work in Task Form Fields, i.e., you can't have lookup type field in task form. If you do, you may see "SharePoint Designer encountered an error generating the task form" error message.