Try 99.9999% the same. SBS has a maximum of 75 devices or users logging into the domain. If you can install it or use it on Standard Exchange, you can use it on SBS 2k3's Exchange.
We can now use the ADMT tool. See these two white papers for more detail
Download details: Migrating from Small Business Server 4.5 or Windows NT Server 4.0 to Windows Small Business Server 2003:
http://www.microsoft.com/downloads/details.aspx?familyid=1c39e0a0-ac03-43a6-a457-81e1695e5bb6&displaylang=en
Download details: Migrating from Small Business Server 2000 or Windows 2000 Server to Windows Small Business Server 2003:
http://www.microsoft.com/downloads/details.aspx?familyid=fa187d1e-8218-4501-9729-222bd8ebb64c&displaylang=en