Recently Office 365 added a new tool to allow customers to switch plans in the middle of a subscription period. I wrote about this here. We had an issue with switching a plan if you were using a custom domain and we've corrected that now. You can Switch Plans even if you have a custom domain. Read the online help for Switch Plans here.
While we had this issue you could still Switch Plans if you were willing to remove your custom domain from your Office 365 subscription and re-add it back after changing the subscription. Now you can just upgrade without the hassle.