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.
@Sakshi - How about the 'Use an existing Adaptive Form' option that is available under Adaptive Form Container properties? Would that not help achieve the use case for simple forms? Although, you might want to emphasize on the pros/cons involved and whether it is recommended.
@Anshika, yes that is a good option. Its good for similar looking templates. Here it will work as reference, that is, you would only be able to author (in editor) the original form. The form with 'Use an existing Adaptive Form' option, is displayed in authoring in non-edit mode. So it will be a little tough to style the form as per the template, if that is not the case you may use this option.