If I now convert my wallet into an HD wallet or a Descriptor wallet and something goes wrong, can I still use my old wallet.dat file again if I copy it to the wallet.dat location via linux termial?
Yes, but it's better to use the proper import method rather than manual copy of the wallet.dat file to the wallets directory.
Use the command:
bitcoin-cli restorewallet "wallet_name" "path_to_old_wallet.dat_file_backup"Should I update my wallet.dat at all? I hardly ever generate new addresses. Or do I have a disadvantage if I leave everything as it is?
What would you recommend?
I'd recommend to migrate to a descriptor wallet.
The main reason is because legacy wallet format support will soon be dropped by Bitcoin Core.