As you think, it is necessary to change URL. There is a site on which there is a forum and blog: www.site.com/forum and www.site.com/blog. I think to make www.forum.site.com and www.blog.site.com. Whether it is necessary to do it?
since you are adding forum or blog to the existing site, either was is just fine. subdomians are considered as seperate entity where folder extention will be considered as part of the same site.
I suggest you go/stay with sub directories instead of sub domains, it is because, I saw Matt Cutts in a video where he mentioned that, Google like to see sub domains any more and Google now prefer to see sub directories.
Although both would be ok, I would select the "sub-directory" first. This will have a major impact on page count, traffic and the overall Page Rank of the primary site.