I want to do this because I want to convert a 2013 wallet into the new format. And I want to do it with testnet coins first, so I don't screw up with real coin wallets. But I need to download Bitcoin Core 2013 first to create a 2013 .dat wallet file. Then I want to be able to migrate it into the new format, and then create a watch-only wallet for an online node in another laptop.
I'm a bit confused here. If you already have the 2013 wallet.dat file, why would you create a new wallet file?
You mean that is for a testnet wallet.dat file? If that's the case, you don't need to do that since you already have a 2013 wallet.dat file. You can make a few copies of that file and do an experiment to see if that wallet would upgrade to the latest version of Bitcoin Core.
So having an extra device is needed, or VirtualBox is fine to do some experiments if it can upgrade the old 2013 wallet.dat file to the latest one.
Just in case it fails, you still have a few backup copies, and you can repeat the procedure to a lower version of Bitcoin Core until you upgrade it to the most recent version before going to the next step to make a watch-only wallet.
If your purpose is to upgrade your old wallet.dat file the newest one; you don't need to use the existing blockchain on the same device. Prune should work if your purpose is to check the balance of that wallet.