Hey Katharina555,
We'd be happy to answer your question. If you were to log out of your current Apple ID and log into another one, any contacts, calendar events, reminders, photos/videos, messages in iCloud, and more associated with the account would be removed from the device unless you choose to download and keep a copy of this data locally during sign out.
Also, any apps currently associated with the Apple ID you have logged in at the moment, would still require the login details to that account when updating. If moving to a different Apple ID, it'd be best to delete any apps you've installed and redownload them under the new Apple ID instead to avoid this scenario from playing out in the future.
In the future to avoid any headaches, it'll always be best to change the Apple ID email address if you no longer use the one on file instead of creating a whole new Apple ID. That way you don't run into the issues described above.
For more details on signing out and signing back in with a different Apple ID, check out: Sign in with your Apple ID - Apple Support
We hope this helps out.
Take care!