English can use "upgrade" as an intentionally underspecified one-word label. On, say, a paywall, Spanish would naturally use a construction like "pasar a [otro plan]," and French “passer à [une autre offre].” The preposition a/à is how that particular meaning is expressed in those languages.

So it isn’t randomly translating “upgrade" as "upgrade to." The context tells you that "upgrade” means "move to a different plan," rather than "update the software."

You're right about the random capitalization though - it wasn't intentional and we'll edit that bit to avoid confusion.