If there are four Sims and four Hobbies in the house, and all four Sims already occupy a hobby, you cannot select anything. There aren't more Hobbies than 4 except for the piano - which I do not own, and did not plan on buying, especially not specifically to finish a quest... to unlock a new hobby. 🙂
Swapping would be possible if at least one of these hobbies would be 'unclaimed'. Otherwise, if you try to do that, you receive a message that "Sim X is already on that Hobby". And, as I mentioned, four sims already have four hobbies, hobbies cannot be 'abandoned' without clicking on a new one first, which again - is not possible. Hence, blocked.
