If you change your cards, you will most likely reset the credit age for that account. This has happened to me twice. Once the credit card company offered to upgrade my card and I accepted. The other time, the card changed provider bank without asking me. Both times they reset my credit age. In the first case, I asked about this and the person I talked to at the bank said they didn't think it would affect my credit score. Whoops.
You can call the credit card company and ask, but there's no guarantee the person you talk to will know the answer--they don't work for the credit rating agency.
========== EDIT ===========
Looks like it does happen more often than I thought. I don't know how you can find out for sure whether it will happen for you, though.