Learn English – Is “switched” always used as a verb

Parts of speech are categories, their members sharing various properties. One of these properties is the functions that the members can perform. These functions are relations, and each should be capable of coming before of. For example, in faculty office, faculty is a dependent (more specifically a modifier) of office. It generally makes no sense to say that something is a noun of or verb of.

If we look at families. Man and woman are categories (like parts of speech). You can see a man or woman outside of a family situation and generally still put them in the right category based on various properties such as facial hair, breasts, size, voice, etc. One of the properties of men is that they can function as 'husband of', 'brother of', 'parent of'. Women can be distinguished from men partly in their inability to function as 'husband of' or 'brother of', but both men and women can function as 'parent of'.

Back to words, the members of the category of English nouns share a range of properties including (typically) inflecting for number and ability to function as 'subject of' or 'object of' verbs. Adjectives have other properties, like inflecting for grade (tall, taller, tallest) and ability to function as 'modifier of' nouns. Number and gradability are distinguishing characteristics, but functioning as 'modifier of' nouns is a shared characteristic. But we can still distinguish them based on their other characteristics. Only when the word takes on many characteristics of another category (like fun--traditionally a noun--being inflected funner and funnest) would we say that it actually now belongs (also) to that new category.

I would advise you to use an adjective steep instead of uphill as it is more idiomatic when used with the learning curve.

having or being a slope or gradient approaching the perpendicular

[Collins Online Dictionary]

This linked Ngram Viewer and this one show steep learning curve and learning curve was steep are more idiomatic.