I will be playing my first D&D-campaign soon and I want to play as a changeling.
Is this possible?
This is 100% official content for Eberron. Since it is an ebook, that also means we can update content with comments based on UA playtests of the races and the artificer.
Whether or not you are allowed to play as one will depend on your GM. A GM can restrict any number of character options to fit the setting or just based on the table. As such, you should ask your GM if they will be permitting the use of the Changeling.
Wayfinder's Guide to Eberron is only an available choice if your AL league (as of version 8.2) is playing the Eberron Campaign (version 1.1). It is not included as an option in the Forgotten Realms campaign.
This makes races like the Changeling only available in AL games that play under the Eberron Campaign.