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   Let me know if this helps.
Hi! I changed the motherboard of our DELL PowerEdge R420 and now we have no service tag defined. The server is running Ubuntu Linux. What is an easier way to set this?
PowerEdge R420 motherboard replacement - service tag