PowerEdge R420 motherboard replacement - service tag

Cberni,

Did you perform the backup as discussed on this page? If so you will follow this link.

If not then you will need to do the following.

Using Dell Support Live ISO 

1. Boot the system using Dell Support Live. 
2. At the Enter option or letter prompt, press <4>. The MS-DOS prompt is displayed.
3. Type c:, and press . The working drive changes to c:\.
4. Type CD Util, and press . The working directory changes to c:\Util.
5. Perform the following:
   • To change the service tag, type asset/s new_service_tag where new_service_tag is the tag you want to assign to the system.
6. At the confirmation prompt, press

Let me know if this helps.