You don't have to create a separate campaign if you want to change the commission level or set up a special discount for some of your promoters. It happens very often to give a better reward for an influencer or a special discount to his followers, different the one set in the campaign.
There are 2 ways to do this:
From the Promotions section
go to "Promotions" sections
find the promotion linked to your promoter(hint: use search box)
once you find it, click on the menu icon on its row(menu icons in FirstPromoter looks like 3 horizontal bars )
select "Edit promotion"
to change the offer(discount) click on 'Set offer'
to change the rewards, click on top link "Custom rewards" and choose what rewards you want to set
From the Promoter Overview page
You can also see the promotion details on the promoter overview page(when you click on promoter name on Promoters section).
click on 'Customize' button near the "Rewards" and "Offer for friends and followers" label
to change the offer(discount) click on 'Set offer'
to change the rewards, click on 'Set reward'