Transfer a new field from Service Header to Service Invoice

Hi, I have created a field on the service header table 5900 which disappears after the service order is invoiced as the service order gets deleted. Is there a way I can either transfer this to the Service Invoice so it is available after or another way of keeping it?  I suppose one option would be creating a new feature which could work here, but it seems overkill for just trying to save a single field if i don't have to do this. Thanks in advance