Microsoft had first announced the plan-switching option back in August of 2013 however the option was limited to within certain plan families. The restriction has been removed and cross-family switching is possible among the plans. It's even possible to jump from the lowly Exchange Online "kiosk" plan to the more expensive Office 365 Enterprise plan. Microsoft lists the eligible plans that can be switched at this support page. However, going in the opposite direction from a higher priced plan to a lower priced isn't shown as an option per Microsoft's support page list of choices. Check out the full story here.
Microsoft is now making it possible to switch Office 365 plans across "product family" offerings however the customer sees fit, regardless of current plan type.