I'd like to interject a technical bit:
In this topic, people are referring to changing plans. If I want to change from an E1 plan to E3 or vice versa, I can do that at any time, with just a few clicks. That's changing plans. Changing from Small Business to Midsize Business, for example, is a change of tenant. If you want a comparison for an onsite equivalent, it's like changing over to a new AD forest. It's not something you can just do that easily.