Do you need to change platform when arrive at Newton Abbott from Penzance and then going London Paddington?

Yes, when you arrive at Newton Abbott from Penzance, you will disembark and then take the steps leading up to the ticket office. Proceed along this corridor past some shops and cafes, past the next ticket office and down the final steps to the second platform.

Aboard your train to Paddington, you can be seated in either car because both stop at the same platform in Paddington.