Just to be clear, this is for POP!_OS, right? And does it automatically update my POP!_OS installation or is it a completely new distro that I'd have to do a clean install for?
Also, as others haven't said yet, CosmicDE will be available for other distros if they choose to support it, which will be different to now where your only choice beyond Pop was to rice GNOME yourself.
They are changing the desktop environment. While it should be updated seamlessly, I do wonder how the configuration files, gnome extensions and dependency issues will be handled when updating to cosmic. It may be beneficial to make a fresh install