Jeremy Crawford gives a simple answer to this on twitter:
A changeling can, as an action, change their appearance and their voice. There is no requirement that they revert to their true form before adopting another form.
You get +2 AC
The Druid Wildshape feature states:
You retain the benefit of any features from your class, race, or other source and can use them if the new form is physically capable of doing so. However, you can’t use any of your special senses, such as darkvision, unless your new form also has that sense.
While the Warforged Integrated Protection grants:
By RAW, You would get the full +3 AC. This is probably not intended.
The druids Wild Shape states:
You retain the benefit of any features from your class, race, or other source and can use them if the new form is physically capable of doing so.
The warforged Integrated Protection says:
Integrated Protection. Your body has built-in defensive layers, which ...
One thing that I noticed was certain magic items (such as a Band of Loyalty) weren't reprinted, the same goes for the Warforged subraces, which did not get updates or reprints. The Valenar and Aereni (elf) subraces did not get reprinted either, for reasons I am not currently aware at the moment. Hope this was helpful!