Announced back in July, the new Office 365 plans for small- and medium-sized businesses are now available for new customers. However, existing customers with the previous plan types will need to wait until those subscriptions run out before migrating. (And even then, they can push it back if desired.)
These changes do not impact Office 365 enterprise plans.
I wrote about the new Office 365 plans back in July in Microsoft Announces Major Changes to Office 365 for Small and Midsized Businesses and What This Year's Office 365 Changes Mean to SMBs. Long story short, they break down like so:
Office 365 Business. This plan provides a partial version of the Office suite—with Outlook, Word, Excel, PowerPoint, OneNote, Publisher and OneDrive for Business cloud storage—for Windows (or the equivalent Mac Office) on up to 5 PCs per user. But it does not provide access to the Office 365 online services: Exchange, SharePoint and Lync Online. It costs $10 per user per month or $99 per user per year.
Office 365 Business Essentials. This provides the online services—Exchange, SharePoint and Lync Online—for business-class email and calendaring, online meetings, IM, video conferencing, cloud storage and file sharing and more, plus Office Online for the web-based versions of Outlook, Word, Excel, PowerPoint and OneNote. It replaces Office 365 Small Business, and costs $6 per user per month or $60 per user per year.
Office 365 Business Premium. This plan provides everything from both the Office 365 Business and Business Essentials plans. It costs $15 per user per year or $150 per user per year, and replaces Office 365 Small Business Premium.
This chart breaks it all down:
If you already subscribe to a previous Office 365 small business or medium plan, Microsoft says to keep an eye on the Office 365 Message Center as you'll be getting instructions soon. I don't see anything there myself, not yet anyway. (In the Office 365 web portal, navigate to Admin, Message Center.)