External markings are not a big deal, they should have been done that way to begin with.
Perhaps a larger consideration is this "order" serves as a "retrofit" order, requiring everything marked under the old rule to comply with the new rule. That may not sound like a big deal but as things move forward what other actions will be taken that require retrofitting previously constructed and registered drones with new hardware, software, or firmware? What costs will be deemed "reasonable" for such retrofitting, if any limit is imposed at all? What considerations will be given to original design limitations that may prevent compliance with a retrofit order?
Those that have flown full scale for a significant period of time have seen the FAA mandate all kinds of things that required alteration of pre-existing, certificated aircraft with little or no consideration for the expense that would be born by the airplane owners. As drone owners we should be very concerned the FAA will follow the same path with our equipment as they did with full scale.
One more concern is how the FAA has been issuing orders and regulations pertinent to drones without following the NPRM process. It seems that having drones referenced in national defense legislation has allowed the FAA to bypass the NPRM regulatory process.