Many years ago when I flew B727 to La Paz there was only 2 aircraft of our fleet approved to "high altitude" operations. Of course the company paid for a special ammendment on our FCOM and as far as I know besides the Takeoff/Landing chart Analysis there was also a special change in the automatic closure feature of the outflow valve (must close automatically when cabin altitude reaches aprox. 14.000ft) - which was a FAR requirement at that time. Both aircraft were fitted with a switch on the second officer panel which changed the outflow limit altitude in order to allow the opening of the valve in case of smoking evacuation procedures at El Alto Airport. Some years later I've been there on a demonstration flight with one of our B767's and all of them were fitted with the High Altitude switch. Since then I have not flew anymore to La Paz. Some of my fellow pilots who operated there with B737-300 said that all they did was setting the landing altitude in the FMC. Hope this helps.