What you mean Anonymous, is to integrate an additional shop. This is easy to do in the settings under 'my accounts...'. This is how I was able to take over all the books from orellfüssli. However, the pre-installed standard shop cannot be changed. Obviously, the eReader is permanently connected to the shop via the serial number. Now I have to go shopping at my favourite shop via the mobile. The book is then immediately available on the eReader via the shop link.
It's a pity that Digitec doesn't specify the preset shop:-(

Does anyone have a solution to switch to OrellFüssli without having to register with Weltbild?

I don't remember how I went about it. In any case, I was able to switch to Orellfüssli without any problems.

I've just realised that too. This is the third Tolino in our household and the other two were Orellfüssli versions and at least one of them is also from Digitec. It's annoying that it's not stated that these are Weltbild-bound devices, but I thought I could still use them somehow. However, the Weltbild Tolinos don't support Skoobe and so I'll have to return it.