I have a Sawsbuck Spring Form in my copy of Moon I transferred from X, and I was wondering if I could get the same Sawsbuck to change into its Autum Form, for example, by battling with the Sawsbuck in autum.
You can get Sawsbuck to change its form depending what month it is only in Gen 5. From Gen 6 onwards, you have to import the other Sawsbuck forms into your respective game so that it's other forms become available. There is no other way of changing its forms except this. Source: