Glad to know that you are keen to learn and want to spend your time on this decentralized Social Media platform (Steemit). Highly Appreciated!

Can we change the reward settings for the posts we already posted?

No, we can't change the reward settings after publishing a post.

Should we change the reward settings for only the posts we post in steemit-pak or for all the other posts as well for example achievement posts, contests in different groups etc.?

That's just a rule we have introduced in Steemit Pakistan for our users to get better support (Booming Upvotes).
You don't need to set steemit-pak as beneficiariary in other communities or contests.

Thank You!