Windows Server Infrastructure - Should you Migrate or Upgrade

Migration lets you to move the configuration of an existing server to a new server computer. Migrations are often selected over upgrades because the process is less destructive and more recoverable. Depending on the services and functionality that you need to migrate to a new server instance, there will be different requirements and actions you need to take. Fundamentally though, migration can be broken down into three phases.

Pre-Migration

• Installing, running migration tools and identifying any prerequisites. For example, drivers and ports.
• Preparing source server. For example, backing up your data.
• Preparing destination server. For example, ensuring drivers and ports are available.

Migration

• Exporting or migrating data from source server.
• Importing or migrating data to destination server.

Post-Migration

• Verify destination server is running successfully.
• Decommission source server.

Why Migrate not Upgrade?
First of all it provides a transition path from x86 to 64 because a lot of people are still running Windows 2003 32 bit applications which can't be upgraded to 2012 since there's no 32 bit version of 2012. Physical to virtual and vice versa so  it's easy to just migrated to a virtual machine. The full server to server core and vice versa.
In Wndows 2008  you could go to install server core and do all the nice stuff on it.  Now with 2012 R2 you can install it with full GUI and remove that if you want later. But all of these components get into  play when you look in at transition paths. Plus it's a clean operating system.

If you are still running Windows 2003 boxes chances are you've got them running for a long time, a very long time. And what happens when you have a system that you keep running for a long time. It starts getting slow. You start getting some installed and removed software on it you may have missed the patch or something. You never end up with a really clean OS. So if you were going to migrate that or do an in place upgrade of that so you are basically migrating all the bad stuff at the same time. So doing a migration instead of an upgrade provides you that opportunity to do a clean system installed and the most important part of that for me specifically is to reduce the risk of the down time. If you are doing a migration your source server is still available so if your migration fails halfway through not a problem. Turn the service back on wipe the machine that was your target machine and then from there on you can try it again figure out what went wrong and fix that. Is it a script is it network bandwidth whatever the problem might be and at that point you can just start going at it again. So the tasks are performed while your service is still live so you have that back out scenario going for that.