on a higher floor vs upstairs

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

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

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

Other synonyms of on a higher floor include up the stairs.

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

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

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