I have a my.wallet file created by bitcoin-0.8.5. I reinstalled the PC and want to open it again and perform a transaction. I have the old installation file bitcoin-0.8.5-win32-setup.exe. However, the client says that an upgrade is needed. How to migrate the wallet to a new client? Or, is there any way to avoid downloading whole blockchain and open it in a different client?
Try opening the old client and get the private keys from the wallet. Then, to avoid downloading the whole blockchain, get an SPV client like Electrum and import the keys.