Yes - Yes you can migrate from sbs 2000 to a regular domain 2003 (without any transition pack)
Trust me it is possible and we have recently done it. The procedure is very simple. Read this article How to prepare the SBS domain for an additional R2 Domain Controller first and add addition R2 Domain Controller to your domain. This article
Creating a trust relationship between two Small Business Server 2000 domains will also help you lot.
After adding addition domain controller to your domain wait for full replication (It is nice to wait for at least 24hr, now shut your SBS domain controller down and size all roles from it. This article will help you in this process "How can I forcibly transfer(seize) some or all of the FSMO Roles from one DC to another?"