Primarily, the electrical system, floors, plumbing, plaster, etc., whatever is to be renovated, cannot be renewed. Or does the uncle have to grant us access to his rooms, to which he has a lifelong right of residence, so that we can renovate them as well?
Of course, you can renovate as you please. However, the right of use must not be restricted. Announce the renovation with a date and carry it out. Just like with the renovation of a rented property. Offer him alternative living space for the duration of the renovation – that should be possible on the property/house.
What happens to his furniture, etc.?
You have to take care of that. Cover, move around, etc.
Does the right of residence also mean that he can put obstacles in the way of the renovation/modernization?
No
If I wanted to make a room out of the bedroom and the small kitchen (take down the wall), can he prevent that?
Yes, you may not do that. Or do you want your neighbor to tear down walls in your house? It’s comparable.
Or is the condition of the "apartment" not allowed to be changed against his will for life? This is not noted in the right of residence document.
The right of residence covers an area X = rooms a,b,c. You MUST leave these rooms to him, but you can influence the condition = renovate.
Does anyone know about this? Are there relevant court rulings? Or how can one deal with an uncooperative uncle? It’s one thing that he does not want to give up his right of residence because it is a family house/was. But that he now puts obstacles in our way and insists on keeping it as it was 60 years ago. Is he allowed to do that?
He only has a right of residence. Comparable to an unterminable rental contract where he only pays the additional costs. You can handle the rooms the same way. Renovate, renovate, etc. The uncle has no way to prevent you from doing this.