When we switch a sentence from active voice to passive voice, it is important to identify the doer of the action and the object or complement of the verb. The subject goes first in active, whereas in passive the object appears first. In the first sentence, "The menu" is the object, accordingly the sentence should be:
The menu was changed by the head chef to honor the restaurant's foreign guests.
In the second example the object is the living room:
My living room was decorated by my mother in shades of lavender and teal.