does anyone know what happens to your credit when you to convert an existing credit card to a new product between the same issuer?

for example, have a chase UA visa closed and credit line moved to open a chase CO card... or like BOA Student Plat Plus converted to a BankAmericard(essentially closing the student plat plus card)

what impact does this show on ur credit report? why i ask is because if it's my oldest credit card with 10 yrs, if i convert it to another product does that mean it's treated like a new card with 0 yrs of history? cuz that will essentially drag down my avg CC history and affect my overall credit score right?