I think all of us were waiting for the possibility to switch between plan in Office 365. Since December 2013 we can switch the plans with all Accounts and Settings.
But you can´t switch from every Plan to every Plan. You can change in the same service family; for example form Office 365 Small Business to Office 365 Small Business Premium oder vom Enterprise E1 to E3. Moreover you can switch between an standalone Plan to an normal plan; for example from Exchange Online oder SharePoint Online to Office 365 Enterprise E1. At least you can change to a different service family like from Small Business to E3.
A list of all possibilities to switch plans and an instruction How to Switch you can find here: http://office.microsoft.com/en-001/office365-suite-help/switch-to-a-different-office-365-plan-or-subscription-HA104031833.aspx#_Plans_eligible_for