It happens when you change your plans to the next level. Here the next level in the sense, transferring your plan from an existing shared hosting account to a reseller account or to a dedicated server plan. Like the same it happens when a reseller account has to be changed to a dedicated server.
And the server is never changed if you are just going to upgrade your Fundamenta Romeo to a slightly Husky Romeo and like the same if your Husky Romeo wants to become a Jumbo Romeo.
This is applicable to all category of plans, means the Reseller Accounts and Dedicated Servers.