It's mandatory to remove Forumotion's ads before adding yours because many ad platforms limit the number of ads on each page in their terms of use.
For your users, it can be really bad to display too many banners too. And Google launched a filter to penalize website with too many ads on web pages (SEO).
Yes you will first need to buy credits and use them to remove forumotion's ads. Just remmeber you will have to make sure you remove them every year, as if you do not keep our ads disabled at all times, it could lead to problems like c driko said.