Paramount didn't "buy" Dynasty, not as such.
Spelling Productions has, over the years, made various deals with various studios on various Spelling properties. As the rights that those other studios have come to reach their expiration date, the rights revert back to Spelling.
Since Paramount has picked up Spelling Productions, those rights now revert back to them. That's why, for example, "Beverly Hills 90210" had the pilot episode put out by Artisan (a studio now owned by Lionsgate), but Artisan never had time to put out season sets: they knew their rights would expire before they could put out enough seasons to make it worthwhile. So Artisan just sat on the property while waiting for their rights to expire and it would revert back to Paramount.
Although Twin Peaks is not a Spelling item, it was in the same situation where Artisan's rights were expiring after they put out Season 1, and then the rights were close to reverting to the new owner (coincidentally, also Paramount).
So Dynasty, which is also a Spelling item (co-owned by them, actually) will revert to Paramount soon without them spending any new dimes on aquiring it. Once it is offcially theirs, they can get to work on producing new season sets for it. Don't know exactly how long that will take, or on what date that the rights revert to them, but it may take a while (a la Twin Peaks Season 2). Fox is just in a bad spot with Dynasty, that's all.
Just another "business as usual" day in Hollywood, I'm afraid!