In Black/White, the PID algorithm were remade and IV/nature has no correlation anymore. You can manually edit IV and nature without worrying about it going unlegit. There is no way to prove if it is legit IV or not as far as i know. PID only affects gender, shininess and ability. Thats why your tick on shiny wouldnt always work because of a new algorithm for the Black/White. Also if you only intend to change his ability, but not his gender/shininess its better to leave the PID field BLANK. PokeGen automatically generates the PID for you. DW abilities also has no correlation to PID. DW abilities simply "overwrites" it. So your safe to change it to whatever DW ability it has without worrying about a new PID. Do note that PokeGen makes the PID based on the algorithms from the last gen!
@FlashX007: PokeSav was awful because it didnt have the ability to make pokemon look legit as compared to PokeGen when Black/White came. You wont be able to go wifi with pokemon made with PokeSav unless you manually create the PID to make them look legit. And PokeGen is easier than PokeSav in terms of just creating a pokemon. PokeSav is harder to create legit pokemon now.
EDIT: And if you really want to make them look legit with the PID values for the shininess/gender/ability, then it will require a GOOD amount of your free time where you use 2/3 of it to learn about hex-editing and how PID works.
EDIT2: And this should be in tutorial/FAQ thread. Not to mention that there is already a guide for this there that has pictures and offers other options than just creating a pokemon from scratch.