I just got my Mavic back after a month wait, because of a hardware Gimble error.  I can see that the aircraft has a new serial number, and when activating, I went through the procedure of a completely new aircraft. This also included the part where I am told that I have 48 hours to buy the DJI Care Refresh plan. But I bought the plan for the old Mavic that was sent in for repair. How do I get the plan registered with the new aircraft?

email them
djicare@dji.com

Sometimes you also have to send proof of purchase of your DJI Care Refresh.

Hi Steen, please refer to the link below to check it first, see if it is already linked to the new aircraft. If not,  like arcyenom mentioned , send an email with the case number and new SN to djicare@dji.com. we will deal with it.
http://www.dji.com/service/djicare-refresh