Is it possible to change a multiple choice form to single choice?

It's not possible to change a multiple selection field to a single selection. You can add options to your single selection field but cannot change its function to be a multi select field.

If you are referring to the form layout, you may change the form layout setting from the form settings to display all the questions in one page. Please see the screenshot below: 

Let me know if you have any questions or if you need further help.