The crossgrade replaces your Cubase license with a Nuendo license, and you’ll no longer be able to run Cubase unless you buy Cubase again.
Nuendo is a post-production tool and isn’t intended to be a “Cubase+”. If you primarily make music, I would strongly recommend you to stick with Cubase unless you can’t live without Nuendo’s additional features. It’s definitely a good idea to download the trial version first.