it would be easiest to just swap the lines at the carriage. You would have to make up new hoses but in alot of cases the lines at the control valve are pre-formed steel lines for the attachment functions and cannot be moved. If the lines in the carriage are steel tubing than that would make moving them at the carriage difficult also.
If this is not the case then you might be able to move the lines at the control valve, or possibly move the connections around at the bulkhead junctions at the front between the mast and frame, but that might be limited due to how the hoses are ran into the mast array and not the best option.

Without knowing the exact configuration you have there it is hard to give good advise on how to switch the functions. You'll just have to figure out the best point to switch the lines around.

As for as relieving pressure before disconnecting lines?
if this truck has the operator presence system and has solenoids on the control valve then just turn on the key, activate the seat switch, do not start the engine, pull on the levers and this should release any pressure on the lines.
If just a standard hydraulic system (old style) just pull the levers in both directions and it should release the pressure.