As others have said, if you have a stem with a removable face plate, this is not too difficult. Drop bars and moustache bars take the same brakes, so there are no new components you need. The complication is "without messing up any of the cables." Depending on the brake levers, you may need to remove the brake cables to get to the bolt that secures the clamp to the handlebar. Alternatively, you may need to loosen the cable pinch bolts to create enough slack for you to work around the cable. For the bar end shifters, you almost definitely will need to loosen the cable pinch bolts (i.e. where the cables connect to the derailleurs) to create enough slack. For both brake and derailleur cables, when you loosen the pinch bolts you may find that the cables have frayed at the bolt and will need to be replaced, anyway.

All that said, replacing cables isn't hard, especially with bar-end shifters, and especially if you use friction shifting on the rear derailleur. (Setting up for index shifting is a bit more fiddly.) As long as the cable housing is in good shape, all you would need are the replacement cables, a decent pair of cable cutters, and crimps to keep the ends from fraying once the cables are set up. Re: instructions, see the Park Tool website for instructions.

Mike