Learn Portuguese (Portugal): Ela vai mudar de emprego? Eu já o sabia.

In Portuguese, the verb mudar followed by the preposition de is used when you are switching one thing for another of the same kind. You will see this in common expressions like mudar de casa (to move house), mudar de ideias (to change one's mind), and mudar de roupa (to change clothes). Without de, mudar usually means to physically move or alter something.

What does the o refer to in Eu já o sabia?

The pronoun o acts like "it" in English. It refers back to the entire previous sentence or idea: the fact that she is changing jobs. While you can often just say Eu já sabia (I already knew), adding the o makes it sound a bit more complete and is very common in written or slightly more formal Portuguese.

Why is the pronoun placed before the verb (já o sabia) instead of after (já sabia-o)?

Certain words in Portuguese act as "magnets" that pull the object pronoun to the position right before the verb. The adverb já (already) is one of these magnet words. Because já comes right before the verb, the pronoun o must jump ahead to join it, making it já o sabia.

Could I use the Preterite tense (soube) here instead of the Imperfect (sabia)?