I guess you heard "we are working on that" already, but even internally we are pushing community team to improve what we spot as sub-optimal :)
I have to agree I liked separated KB and Forum, but neither one was perfect even before.
Left side of the search bar is where you can select if you'd like to search in forum, in KB, or everywhere.
Hint: if you search for something we can not find (like "cabigwbkjsdb") then "Click here to clear filters and perform a fresh search" will appear alongside of note we have no match, and click on that sentence let/brings you to advanced search menu, with more options..
Alternatively you can select at the top under Community logo, where you can get, to Forum or Knowledge Base. And pull down menu will let you jump into specific section.
Here in Forum, you can use Labels (as mentioned by Debbie) to refine which topics you'd like to get.
That vertical 3-dot icon will let you then to subscribe to specific labels = feeds.
Giving you notifications only to forum posts about topics you are interested in and so selected (FortiTokens for example).
However that Labels system has one caveat.
Users should post their questions to respective topics/labels. As otherwise it will become one big pile and unmanageable.
Labels categorization, Labeling and proper placement is also what our community managers are handling and what we are trying to improve.
Hope this helped at least a bit to relieve stress from new system.
Tom xSilver, planet Earth, over and out!