thx ranichigo,
The upgradewallet command will be necessary I guess, I'm first re-syncing the whole blockchain to have the best starting situation to do all of this.
Others told me I could see the balances, but I don't know where I can see these old adresses. (I only have a password written down on a text file and the .dat file, all other info is lost).
To go to cmd in bitcoin core, I just need to go to settings I presume? Or do you mean command on a linux device? (I'm using windows as you might have guessed

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thanks for all the help so far
I don't know the address or anything, I just have the .dat file itself from a backup (all the other info was lost) and the passphrase.
I'm willing to download the full blockchain again, that's not the issue, but I was wondering why there's no good info out there to really restore an old wallet file.
Even if you do know the address, if it's an old backup, it would be better for you to resync the whole blockchain to ensure that you'll be able to get all the coins in the addresses within your wallet.dat.
So if I have 2011.dat for example, and rename it to wallet.dat and place it in the bitcoin folder where the new wallet is then restart bt core then it should work?
I heard older wallet files were incompatible since 2012 or something.
Yes. Do keep a backup of your wallet.dat though (both your current and 2011). It's true that the wallet.dat files had its structured modified slightly but its not wholly incompatible. The client would be able to identify and upgrade your wallet.dat to ensure compatibility. Using cmd, navigate to the folder where you have your Bitcoin Core installed and start it with -upgradewallet flag.