Topic: 2 x ff800 QUESTION

Hi guys...
i have 2 fireface 800's setup but would like to make my newer one (the one with the newest serial number) the main unit as my second unit (with an older serial number) has a dodgy output on channel 5.....rme states in the manual that the devices are labelled in accordance with the oldest serial number being 1st..newer serial numbers last..or in my case 2nd....is there any way for me to change this around????.....
if not, whats the address i need to send my old unit to for a bit of a service...also any idea how much this would cost??
thanks,
tris