So can I turn this watch-only wallet subsequently (=now) into a full wallet?
Simply by importing seed or private key?
Or do I need a full new installation.
The software is already installed on your system. You don't need to install it a second or third time. A watch-only wallet always remain a watch-only wallet. It doesn't contain private keys to broadcast transactions.
Later I want to toggle this wallet occasionally from watch-only to full and back.
Is this possible without fiddling with seed and/or private keys?
Do you plan to do that on the same device? You don't gain anything in doing that. Watch-only wallets are useful in combination with a cold wallet installed on a different offline machine. Using both on the same system brings zero benefits.
I assume you have bitcoin in your wallet already, right?