I'm planning to upgrade my free membership in projects part to premium annual plan which is 299$. So, what will happen if i decide later to upgrade to enterprise plan, am i going to pay as a new subscription (i.e. 599$ as it is from scratch), or i will pay only the difference between the two plans.

The other question is that, what will be the plan period in case of upgrading from premium to enterprise later on? is it going to be another year for the enterprise? or it will be considered as only extension of the current plan which is premium plan in this case?

Thanks in advance