Migrating form Windows 2000 Domain to 2003

[rocketphoenix]

Hi,

We are planning to migrate our windows 2000 domain contoller to windows 2003..
The main Scenario is that we have a Root domain and a child domain.Our new plan is to restructure/migrate and have a single root domain.
ex.) our domain is 123.ABC.com (Windows 2000 based domain)and we want to migrate it to 123.com (Windows 2003 based)
How will we migrate without affecting the accounts (SID's)and policies that we'v been created on the 2000 environment?
Could someone help me to design and provide me the strategy on how to implement this thing.. or a whitepaper with almost same scenario..

Hoping for your help..

thanks
phoenix