If we upgrade to quantum resistant algorithm...

[01BTC10]

Does bitcoins in older address will be protected or we will need to transfer them into a new wallet?

[1656817111]

1656817111

[DeathAndTaxes]

Assumming the risk of QC is real and an effective threat ....

you would need to transfer funds to new QC resistant addresses.  Funds remaining in "legacy" addresses would remain vulnerable although those which have never have funds spent from them (public key unknown to the network) may be immune.  The same transition may be needed if a serious cryptographic flaw is found in SHA-256, RIPEMD-160, or ECDSA.  The odds of this being necessary in the next decade or so is probably very low. Even when such a transition is made the risk probably will be only theoretical and the new addresses just a precaution.  Modern vetted cryptographic systems tend to not just go from secure to fatally flawed overnight.