Using two different Bitcoin versions without messing up blockchain data

I'm looking to run an older version of Bitcoin Core from 2013 alongside my regular Bitcoin Knots 28 setup, which has the full blockchain all downloaded and validated (that took a ton of effort). So I'm wondering how to get this old version of Bitcoin Core installed without messing with my existing files. My plan is to just set up the 2013 version in a separate folder, assuming it will pull the block files from the /.bitcoin directory. But I’m worried that instead of using my current files, it might mess them up or force me to revalidate or even redownload the whole blockchain again, which would be a nightmare. I really want to do this to convert a 2013 wallet to the new format. I’d like to start with testnet coins first, you know, to avoid any issues with my actual wallets. So, first thing’s first, I need Bitcoin Core 2013 to generate a 2013 .dat wallet file. After that, I plan to migrate it to the new format and then create a watch-only wallet for an online node on a different 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.

Your concern is correct. Looking at change log on older version of Bitcoin Core, there are several change to format of block database format and chainstate. Just like @BitMaxz, i'm somewhat confused and don't know your exact goal. But i'm fairly sure even older version of Bitcoin Core let you create wallet.dat file without performing sync/IBD.

Are you referring to to the "migrate" command in your other post? If so, that command will automatically create a backup of the wallet.dat for you, it'll be saved in the same directory of the upgraded wallet.dat file as ".bak" file. You can also create your own copies before migrating if you prefer. And it will not touch your blockchain, other datadir files and settings if you're worried about those.