Yes, on 6.2 we have removed the changing of template. Here we have introduced editable templates, which have more power than static templates (such as components by reference, - defined in structure layer of editable templates).
But if you still wish to change the template of an already created form, you can do it using crx/de: (though this is not recommended, and some components might not work, if they depend on page level configurations)
1. Say 'test1' is the form you want to change template of.
2. Create a form say 'test2' using the desired template.
3. Go to crx/de, navigate to node '/content/forms/af/test2/jcr:content/guideContainer', delete it and save.
4. Navigate to node '/content/forms/af/test1/jcr:content/guideContainer', copy and paste it under '/content/forms/af/test2/jcr:content' and save.