How can I make my old wallet.dat work in a newer client? Im having trouble with my wallet.dat file in the old bitcoin qt client the blockchain won't update because the computer is old and outdated so I'm unable to send the bitcoins that I have in the wallet to another address, so I downloaded bitcoin core onto my other computer and copied the wallet.dat file and replaced it and then in command prompt I rescanned and still nothing, could really use some help if anyone has any idea
You may need to export the private key of your old wallet in the old QT client, and import it into the wallet in your new QT client. The steps to do this are:

Exporting the private key of your old wallet, containg the funds. To do this, in your old QT client, click Help > Debug window and go to Console
There, you can use the command dumpprivkey command to export your private key. KEEP THIS SAFE, anyone with access to this key can do whatever they want with your wallet.

Now we import the private key into the new wallet and up-to-date QT client. To do this, repeat the previous steps: click Help > Debug window, and go to Console again. Then use the importprivkey "KEYHERE" command, where KEYHERE stands for the private key you exported.

The client will then scan the blockchain and, if it finds transactions pertaining to your old address/wallet, it will replay them, and you should dispose of your old funds.