Apparently, Sonic Underground won't be on the Sci-Fi Channel for just this month. It will also be on at LEAST through November as well. In fact, Knuckles' first appearance in the show (“Friend Or Foe”) will be shown that month.
Strangely enough, on the 29th of November, they show “Wedding Bell Blues” again, followed by “No Hedgehog Is An Island” again. Without even showing “Flying Fortess”, which seems really odd. As for if it'll continue into December is as of yet unknown.
Sources: The Sonic Foundation and Sci-Fi Channel.