Old banknotes

I have a bunch of older 1000Y and 500Y banknotes from the 70's ("Series C" I believe).
Can I still use them, or do I need to go to a bank and change them into new ones ? Hopefully, they are not totally valueless...

Thank you for your help.

Both of them are still valid, though
(1) vending machines cannot accept them
(2) as they are not in circulation for a long time and people are afraid of counterfeit notes, shops may not accept these old notes.

You can check the currency in use here:
http://www.boj.or.jp/en/type/list/yuko/okane.htm
(Valid Bank of Japan notes shows the 500yen and 1000yen notes you mentioned.)

I also checked if these notes has more value if you take them to the shops handling old coins/notes but unfortunately they do not have much value and sometimes you may get less than the face value, so the best option seems to be taking them to a bank for exchange to the current bank notes/coins.
You can ask for exchange at the bank at Narita airport.