The trailer is fitted with a set of axles that are already on little rails so they have perfect alignment. The trailer factory then welds the little axle rails to the bottom of the trailer frame rails.
To "flip" you get the axle rails "unwelded" from the trailer rails, insert a new fat rail of the height needed each side and weld that all back on. This preserves your axle alignment on their original little rails.
You might have to add wire to the brake wires to get the wires long enough for the new distances.