Question : Problem: downgrade to 2003 and later to 2008

server 2008 is becoming the new standard and most dell servers only come with server 2008 with a 2003 downgrade. if you downgrade the installation to 2003 and say 3 years later everything is server 2008 can you upgrade back up 2008 without breaking licensing ?  

Answer : Problem: downgrade to 2003 and later to 2008

Yes, you bought Server 2008 with the box.  You also need to make sure the CALS you bought are for Server 2008 and upgrade any 2003 CALS to 2008 CALS to be in compliance.