Flash Catalyst is not an "upgrade" from Flash, nor will it replace Flash Professional at all. Flash Catalyst is a new product. No word from Adobe yet on when it will become available, for how much, and if it will be a part of any of their Suite packages.
Personally i feel that FC should be included in the Creative Suite ... especially the Premium Design suite. Most designers I know use Indesign, Photoshop and Illustrator religiously but barely use Flash and Dreamweaver (except for very basic web projects.) For me personally this is due to coding and scripting. I cannot express my ideas properly due to my lack of web language capabilities! FC will really help (not entirely solve) that problem.
I don't see this tool being as beneficial to the already accomplished web designer however. If they already have a great understand of actionscript, I don't see why they would feel the need to purchase and learn a new software. Kinda the way html coders view Dreamweaver. They may however use it for quick prototypes and obviously when collaborating on Flex projects.
Just my 2 cents.