Running 2 different versions without screwing up blockchain files

[takuma sato]

I would like to run a version of Bitcoin Core from 2013 along with my normal Bitcoin Knots 28 that I have installed and has the full blockchain downloaded and validated (which took a lot of work to set). So my question is: How do I install an older version of Bitcoin Core and use the existing files, but not have them screwed up?

My idea is to simply install Bitcoin Core 2013 version in other folder, and it should use the /.bitcoin folder to get the block files. But my concern is that instead of using the existing files, it's going to do something to the files or something that forces me to validate or even download the entire chain again which is hell.

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.

[BitMaxz]

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.

[ABCbits]

My idea is to simply install Bitcoin Core 2013 version in other folder, and it should use the /.bitcoin folder to get the block files. But my concern is that instead of using the existing files, it's going to do something to the files or something that forces me to validate or even download the entire chain again which is hell.

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.

The first time you run version 0.15.0, your chainstate database will be converted to a new format, which will take anywhere from a few minutes to half an hour, depending on the speed of your machine.

The file format of fee_estimates.dat changed in version 0.15.0. Hence, a downgrade from version 0.15.0 or upgrade to version 0.15.0 will cause all fee estimates to be discarded.

Note that the block database format also changed in version 0.8.0 and there is no automatic upgrade code from before version 0.8 to version 0.15.0. Upgrading directly from 0.7.x and earlier without redownloading the blockchain is not supported. However, as usual, old wallet versions are still supported.

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.

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.

[nc50lc]

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.

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.