Learn Spanish (Latin America): Esa mujer se transformó en un buen ejemplo para nosotros. - That woman transformed into a good example for us.

The verb transformarse is pronominal here. Without the se, the verb transformar means to transform or change something else. By adding se, it shows that the subject underwent the change herself (she transformed or turned into).

Could I use a instead of en to express "transformed into"?

No. Verbs of profound change that are followed by a noun—like transformarse and convertirse—always require the preposition en in Spanish to express "into." Saying se transformó a is incorrect.

Why does the sentence use buen ejemplo instead of ejemplo bueno?

You could say un ejemplo bueno, but placing this kind of descriptive adjective before the noun is very common and sounds a bit more subjective. Just remember that when bueno is placed directly before a masculine singular noun like ejemplo, it drops its final "o" and becomes buen.

What would it mean if I used por nosotros instead of para nosotros?