Based on the questions you are asking, I'm betting that you haven't done much in the way of research into this conversion, its requirements/limitations, etc. AA has a ton of quality information on their site (see specifically the FAQ section) that will help to address your concerns, but in brief...
a) As to your handguard, it depends on what handguard you currently have as to whether you'll need to move to a different one or if your existing handguard will work with the AA piston conversion kits. See the FAQ's I mentioned above as there is an extensive list of handguard systems that have been proven to work with the AA conversions, as well as a specific list of those that will NOT work. All others not mentioned in the list are likely to need modification to work, but they can be made to work depending on whether or not you want to go that route or just replace the HG to avoid the headaches.
b) Your current bolt CARRIER will NOT work with the piston conversion. Your bolt will be used along with the AA piston carrier supplied in the conversion kit so that you don't have to worry with headspace/fitment issues switching to a different bolt.
c) The AA kits will work unmodified on your 6.8SPC/II, but you'll have to change the adjustments on the gas block to ensure proper function/cycling with your can installed.