I am running a rise of the rune lords campaign and one of the members picked a qinggong monk and I looked at the class and i cant grasp the idea if he switches out one of his abilities and uses another one can he still pick his old ability after the duration is over or during the duration. If he chooses barkskin instead of slowfall does he replace it forever or can he change from diffrent abilities?
Think of the quinggong monk as an archetype with a lot of optional class abilities. There is a list of abilites that you can trade, and a list of abilities that you can get in exchange.
But yes, once the trade is made, it's permanent.
The only caveat is that you can trade a higher level class feature for one you gave up. For example, if you traded Slow Fall for Barkskin, but later down the road you realize you really really want Slow Fall back, you could trade, say, High Jump for it.
Also, when you pick up an old ability that you gave up, and then get it later on, it is now as a SLA not as Su. This is important because many spells, abilities, and whatnot that don't work in anti magic is SLA, so be sure to trade carefully.
This also opens up the quicken spell like ability feat, once you get back an old ability later on. If DM allows it, it's a good trick.
Another useful, if late-level ability for the Qinggong monk is that it will eventually be able to learn Blood-Crow Strike at around lvl 14. That particular spell allows you to make your unarmed strikes out to the range of the spell (Range= 100+10/class level). At 14th level, that means you'll be able to FOB out to a range of 240 feet away from you without range increments (Since you've casting a spell), so you can do your unarmed stroke damage out to the range of your buddy elven archer who uses a composite Longbow.
Might take a while to reach that level, but it'd still be fun in my opinion.