At last you have chosen to go to Office 365, your business keeps on growing and at a certain point you reach that critical point in your subscription. You need to migrate from a P to M or even a E plan. In the “old” days you needed to create a new tenant, do a lot of migrations. This is not the case anymore.

1. Switch Matrix, which plans are you eligible for ?

2. How can you switch between plans:

Many plans can be switched using the Switch plans wizard. You cannot use the Switch plans wizard if your subscription has more than 300 users. To access the Switch plans wizard, you need to be a global admin (for Office 365 Enterprise and Office 365 Midsize Business) or an admin (for Office 365 Small Business).

1. Review what happens with your service while your plan is being switched:

• There will be no interruption to your service, and users will still be able to access their data.
• Admins will not be able to use the Office 365 portal.
• Users on your existing subscription will be automatically moved to the new subscription. Note: Don’t reassign user licenses to the new subscription while your plan is being switched; this can prevent the switch from completing.
• The billing on your existing subscription will be turned off once you submit your order and the billing on your new subscription will not start until the switch is complete.
• If you want to use another one of your custom domains for your Office 365 public website address, change the address before you switch plans. If you want to change it to another domain later, you might need to take some extra steps.

2. Go to Admin > Licensing (or Manage and purchase licenses).
3. On the Licenses page, select the plan you want to switch to, and then, under Plan options, click Switch plans.
4. On the Choose a new plan page, select a plan, and then click Next.
5. Walk through the steps in the Checkout wizard. You can add licenses on the Review page by clicking the Edit link next to your new subscription, or change your payment method or frequency on the Payment page.
6. On the Order completed page, click Status to complete your order.
   The Switching plans: status page appears; the switch plans process has begun and you will see this page until your plan has been switched. You can close the Switch plans: status page and come back later without interrupting the switch plans process.

More details on: http://office.microsoft.com/en-us/office365-suite-help/switch-to-a-different-office-365-plan-or-subscription-HA104031833.aspx#_To_switch_plans