We know that the complements of action verbs can be neither an adjective nor a prepositional phrase, but, it seems that that situation changes for linking verbs.
- He was upstairs.
"Upstairs" is an adverb as you know and it is a part of the verb "be", which it belongs to.
So, it has an adverb complement?
- He was at the party.
"At the party" is an adverbial prepositional phrase expressing where he was, and it is also the complement of the verb "was" ?
- I feel at peace.
"At peace" is a prepositional phrase and belongs to the verb "feel". I can't separate "at peace" from "feel" because the meaning is getting lost.
- I was forced to be a bad person.
"A bad person" is a noun but it obviously seems to be belonging to the verb "be".
Do you also think that linking verbs can have adjective, prepositional phrases, noun complements?