May I use an older Navigo card in Paris? I have one from two years ago (the type with a clear plastic holder) and want to just purchase another week on it.

Yes you can re-use your Navigo card from 2 years ago.  Just present the card at a ticket counter and ask to recharge for a week.
 
You can also use the ticket machines to recharge a Navigo with Euro cash / coin / pin protected credit card / debit card.