Capitalization of "used to be"

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For any English grammar whizzes: In the title “She Used To Be Mine”, what part of speech is the word “to”? I’m not sure whether it’s part of a phrasal verb “used to be”, part of an adverbial phrase “used to” modifying the verb “be”, or something else. Instinctively, it looks wrong to me to have it lower case in this title (it’s not the start of a prepositional phrase), but I don’t know how to reconcile this feeling with the style rules.

I’d say it’s this. “Used to” can modify any verb, with the meaning “they did this/had this status in the past but don’t now.” (Or you could say To is part of the verb since the phrase is Used + Infinitive.)

That makes sense. It’s still not clear to me how to apply the style guidelines, however.

It’s being used as part of the verb, not a preposition, so keep it upper case.