I double checked this feature with the team and it should work correctly out of the gate. The one thing we are missing is showing the transfer numbers on the Droplet size tables, and we are going to fix that. We will also add a note on the FAQ, just to make it clearer. I will close this item when we do both of this actions.
In summary, feel free to resize droplets and your bandwidth billing quota will be pro-rated accordingly.
Thanks for sending the question.