upstairs vs on a higher floor

upstairs is a noun but on a higher floor is not a noun.

Word	Noun	Adjective	Verb	Adverb
upstairs	Yes	Yes	No	Yes
on a higher floor	No	No	No	Yes

As adverbs, upstairs and on a higher floor are synonyms defined as:

Other synonyms of upstairs include up the stairs.

upstairs (noun)	on a higher floor (noun)
the part of a building above the ground floor

upstairs (adjective)	on a higher floor (adjective)
on or of upper floors of a building

upstairs (adverb)	on a higher floor (adverb)
on a floor above	on a floor above
with respect to the mind