When does the past participle change form if conjugated with the verb avere?

If the verb avere is in a compound past tense (passato prossimo/trapassato prossimo) and is preceded by a direct object pronoun (pronome diretto-mi, ti, lo, la, li, le) the verb avere must agree. For examle, Ho visto mia mamma. (I saw my mother). This would become chi l'ha vista? (who saw her?) because the verb avere is preceded by the direct object pronoun la.