I saw over on Nicholas's blog post - Nicholas King : EBS/SBS Upgrades & Migrations that he has started to answer some of the questions around system upgrades and migrations.  Below are some snippets of key information he has shared.  Read his blog post for more.  However, key thoughts for me are:

1) no in box SBS 2003 -> SBS 2008 upgrades.  This is a box to box migration process.  If you have SA or FPP copies of SBS 2003 then you can potentially do a P2V and then migrate from the virtual image.  If not, I would consider running a VM of Windows Server to enable you to move the domain at least

2) Moving from one box to another (eg hardware upgrades in the future) are now supported.  This can potentially be used to go from a RC0 build to RTM, but it is not tested or supported, however that does not mean it will not work!!

3) There is a GREAT document linked below that describes a migration in glorious detail

EBS/SBS Upgrades & Migrations

Recently I had several people ask me whether it is possible or supported to do a build to build migration or upgrade from the RC0 of either SBS or EBS to RC1 or even RTM. 

Migration and Upgrade are two words often used interchangeably.  Technically as you know they have some very specific meanings.  Upgrade means to take the existing code base and then lay binaries over the top of, or underneath of (believe it or not this is how Vista does it!) the existing installation effectively taking the old installation and making it the new, whilst maintaining the existing configuration.   Migration means setting up a completely new environment and then transferring the settings, user data and  environment functionality to the new setup.

Upgrades are not something we design, test or develop for either SBS or EBS.  For RTM our research points to the fact that most installations have new hardware on site, and added to that the SBS architecture change from 32-bit to 64-bit prohibits us from doing in place upgrades. EBS is V1 so no upgrade requirement there.

This leaves migrations, I need to be very clear that it IS NOT SUPPORTED from any build to build.  Code can and does change from each of these milestones and we focus our resources on shipping a quality product for RTM.  Now that that's out of the way, let's cover if it's possible..

Yes it is possible to do such a migration.  We do this for our TAP customers using the same migration document that is on TechNET (we add some more details to it to cover the changes we make, but for the most part it is the same doc).  Both products have focused heavily on migration scenarios.  I'll break this into two sections to make it read easier.

Small Business Server 2008 Migration

Migration is one of the killer workloads for SBS 2008.  We've built so much from SBS 2003.  An SBS 2008 migration is based off of a domain join side-by-side configuration where you migrate the user accounts and settings from the old machine to the new using a 21 day trust we create.  It is also answer file based so you'll need to kick off the tool found in the SBS 2008 binaries.  You can read more by following the migration guide found here

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ttfn

David