To allow an air conditioning unit to be turned through 90° requires more than just a choice of fixing points or brackets. Moving from the upright to the horizontal can have a marked effect on performance - refrigerant flow can differ, airflow patterns can be less effective and noise levels can increase.

Unless it's an LG Underceiling unit. They have been designed to operate equally efficiently, effectively and quietly whether wall or ceiling mounted - in fact, they feature a switch that distinguishes between the two extremes and adjusts performance accordingly.