My company has outgrown SBS/Essentials. How do I migrate off?

This answer is high-level because I don't know which version of SBS or Essentials you're running, and because if I explained each step the answer would become huge. You can find documentation online for each of these steps.

First of all: If the user limit you're running up against is the 2012 Essentials 25 users, you can upgrade in place and the existing tools will work with up to 75 users. If, however, you're running 2008 or 2011, read on.

1. Set up a server with a supported version of Windows Server Standard edition and add it to the domain. (Yes, this is allowed--whether you are migrating or not.)
2. Make the new server a domain controller. (Yes, this is allowed and even encouraged--whether you are migrating or not.)

3. Migrate any locally-hosted software and services that you're actually using on the SBS server onto a non-DC server or servers (because installing Exchange, Sharepoint, etc., onto a Domain Controller is only supported in SBS). (Yes, this may involve setting up yet another server with a supported version of Windows Server Standard edition.) Note that:

   • You cannot use the copy of Exchange or Sharepoint that came with SBS. You must buy a full, currently supported version.
   • If you're migrating off SBS2008 or earlier, be aware that you cannot upgrade directly from Exchange 2007 to Exchange 2016. The most recent version of Exchange you can upgrade to is 2013 (because the servers must coexist for the migration). Make certain that your planned upgrade path is supported. (Downgrade rights may be your friend.) I assume the same is true for Sharepoint but my company wasn't using it. (You can, however, migrate to Office 365.)
   • This may sound obvious, but is probably worth stating anyway: Don't migrate any software or services your company isn't actually using (be certain). Make sure you know what software is installed on your SBS/Essentials server and whether or not your company is using it.
   • I am not aware of any supported migration path for the Remote Web Workplace. Transition your users to a more traditional VPN, or see if RD Web Access meets your needs.

4. Migrate DHCP and Certificate Services off SBS (assuming you're using them--be certain).

5. Migrate your file and printer shares off SBS and onto a non-DC server (putting them on a DC isn't recommended).
6. Migrate the FSMO roles off SBS. This will start your 21 day migration grace period.
7. After the grace period expires with no issues, demote/decommission SBS. It's recommended that you format and reinstall over SBS at this point with a full, supported version of Windows Server Standard (because you will no longer be able to run it in the new environment with the FSMO roles moved). This assumes that your hardware is still in support. If so, you can make it a second domain controller, because it's recommended that you have two. (If the disks are huge, perhaps you can install Hyper-V and create a virtual DC on it.) If not, do whatever you do with out-of-support hardware.

This will be a large project and will involve replacing one server with two or more. (Four, in my case.)