You can. You can move from eBook to Paperback or Hardcover, or from Paperback to Hardcover. The cost is the difference between plans. Or half the difference, if six months have passed on your Paperback plan.
Downgrades are simple too. For example, if you are on the Hardcover plan and decide to downgrade to the Paperback, you would be charged for Paperback on your next renewal date, and hardcover distribution would stop.