Okay, Shmoopers, we're going to get deep. Two levels deep.
There are two levels of "memory and the past" involved in this play. There is a) what is in the past from the perspective of the characters in the play and b) references to events that were in the past from the perspective of Aeschylus and his audience, but were in the future from the perspective of the characters in the play. This play was basically an ancient Greek period piece.
There are a lot of references in The Eumenides to a specific historical event that happened not long before the play was first performed. These were the political reforms of a guy called Ephialtes: he took away most of the power of the Areopagus—a council of powerful Athenians.
Questions About Memory and The Past
Why do you think Aeschylus included so many references to (for him) contemporary politics in his play?
Do you think it's still possible to understand Aeschylus's play without knowing its historical context?
Does the play portray change as a good thing or a bad thing?
If memory is unreliable, are the people of Athens likely to keep their promise never to abolish the institutions Athena has set up?
Chew on This
Even without knowing the specific historical context of Aeschylus's play, it is still possible to understand it because its themes are so universal.
The play does not portray change as inherently good or inherently bad. Change for the better is better, change for the worse is worse.