Is it permissible for the son to legally change the name of his deceased father, "Abdul-Husayn," and if so, is it permissible to change it to something other than "Abdul-Rabb Al-Husayn"?

Naming "Abd al-Husayn" is not permissible because it involves servitude to other than Allah, and whoever has been named with this name must change it. The questioner is not obliged to change his father's name "Abd al-Husayn," but there is no legal impediment to doing so if possible. He is not required to change it to "Abd Rabb al-Husayn"; rather, he may choose any good name such as Abdullah, Abd al-Rahman, Muhammad, Ahmad, Harith, Hammam, and the names of prophets and righteous people, while avoiding names that involve self-praise like "Malik al-Amlak" (King of Kings) and "Shah Shah" (King of Kings), and "Taqi" (Pious) and "Mukhlis" (Sincere), as well as ugly names like "Harb" (War) and "Shihab" (Meteor). The evidence for this is that the Prophet, peace and blessings be upon him, changed the name "Barrah" to "Zaynab" because it implied self-raise, and he changed "Shihab" to "Hisham" and "Harb" to "Silm" (Peace).