If you follow the steps on the link above, just creating a new alias, you will still have the same Apple ID as of today. The difference is that you can send and receive email on another iCloud-address as well. The current iCloud-address is still active and that's the address that you will use to log in. This means that you can have several @iCloud.com email addresses to the same account and the same inbox.
Your Apple ID: same as today
iCloud-email address: same as today + the new alias that you create
Your iCloud data: untouched, since it's the same account
Creating an alias is great if your want to keep your data, Apple ID etc. but you may have changed your name or want another email address to share.