Forums have been announced as going at least two years, and that was the main purpose of the development of Discussions. It will not remove the "FORUM" part of the navbar, but replace it with "DISCUSS" and a link to Discussions. Any other changes to the nav bar would be unrelated to this change.
To state what Tupka217 said in a more blunt manner, no, there will not be more free space in the navigation bar. Also, the replacement of Forum with Discussions has been on the works for about 4 years at this point. It was November 2016 that Fandom stopped allowing wikis to enable Forum and forced new wikis to use Discussions.