There also are nobles in the high-ranking noble’s block who are forced to move to a smaller mansion close to the low-ranking noble’s block due to harsh financial situations or ruin. In reverse, there are also nobles who move to large mansions close to the royal castle due to prospering.
Apparently something like that often happens around there because of the vicissitudes of fortune.
Even though I wasn’t aware of those reasons, it looks like the remaining mansions themselves are traded using realtors and such as intermediary.