This is more a long comment than an answer.
There are two questions here, one related to the upgrade and another related to the
About the upgrade. There are several surnames that are a combination of both family names, where the second one is the one that carries the tradition (Ruiz Tagle, Pérez Cotapos, García Huidobro), in those ...
There is a long discussion about the article name in Wikipedia in Spanish. In spite of being a heated and probably un-scientific debate, it shows that both names are used. A rapid check of how are streets named after the emperor (by googling "calle Carlos I" and "calle Carlos V") shows that both are used.
Furthermore, neither Charles I nor Charles V are ...
Emperor Charles V had many realms and many royal and noble titles. So he has different numbers with his name in different Wikipedia lists of monarchs.
As king of the Castilian kingdoms, he was Carlos I.
As king of Aragon, etc., he was Carlos I.
As king of Navarre he was Carlos IV, V, or VI since the list in Wikipedia is messed up. It counts a titular ...
In his role as Spanish King, including Spanish possessions
Charles I, King of Spain
otherwise (in Pan-European context)
Charles V, Holy Roman Emperor
Since he was active in both areas, the different titles should be used in context.
For persons familiar with British history, Charles V and Charles I are 2 distinct persons, both of which are well known ...
I believe your subtext is incorrect - "most popular" is an inappropriate approach to take to correct usage. The correct usage, in either language, should be determined by which of the two roles is most relevant to the discussion.
South and Central America were never part of the Holy Roman Empire, so they were explored and conquered by and in the name of ...