We came here with our Soviet passports. After a while passports were changed in my country, people got new passports. We never applied for the new passports because we did not travel and, so we did not want to spend extra money, especially that my country does not accept dual citizenship. The passports we came with are not in force anymore, even if they are not expired (which they probably are already).
My husband and I had no problems at the citizenship interview regarding the passports (I'll check today to see if they were expired at the time). And, if the Guide says it is OK to go to the interview with an expired passport, then IT IS OK.
What is not OK, is that if you have an urgent situation and you need to travel then you are in real trouble, like my husband was and still is. In order to travel to Armenia he is now applying for Armenian passport, since his name check is still pending for indefinite time.