If the SharePoint list/library is associated with multiple content types then SharePoint out-of-the-box edit forms provide provision to switch to different content types. Attached a screen shot for the same in case I am not clear. 

However, Nintex form requires to be created for every content type. Nintex form in edit mode does not list the content types available in the list/library unlike the SharePoint list/library edit forms. Thereby, we cannot modify the content type using Nintex form.

I require this in upload document scenario where document gets associated with default content type and I would like to switch to different content type but Nintex form does not list them.

Please suggest any solutions if you have come across this situation.