Well, I'm not really sure for a official reason, but along with some folks in Facebook, we came to notice a very funny thing:
The official japanese name of the games is "Akumajo Dracula" which means "Demon Castle Dracula". Now, why would IGA drop this title for some of his games? Simple: Dracula's Castle is not present (in some way) in the games that the Akumajo title was dropped, therefore it would make no sense no name the games as "Demon Castle Dracula". Thus he opted for another title that still represented the series.
Let's check it for ourselves:
-The first title to have the name change was Harmony of Dissonance (named Castlevania: Byakuya no Concerto in japanese). Neither Dracula nor the Castle in this game are real. Dracula Wraith is an evil manifestation from within Maxim, and the castle is a creation from his mind.
-The second title to drop Akumajo was Aria of Sorrow (named Castlevania: Akatsuki no Minuet in japanese). Dracula is not really into it, but the Castle is. We hypothesized that in this case, the name was hidden for the element of surprise, PLUS the Castle is masterless and Soma does NOT inherit it. The castle is ready to be controlled by anyone powerful to claim ownership of it, and Soma was dragged into the story by Arikado, as he himself reveals in the end. The characters call this "Dracula's Castle" out of custom, and from the fact that it was created by his magic, but the castle is in fact, no-one's-castle, waiting for a new Dark Lord to claim its throne.
-The final title to drop Akumajo was Lament of Innocence (named simply キャッスルヴァニア (Castlevania) in japanese). Dracula is not really into it again, and the castle is absent. And like in Aria, Dracula's "reveal" happens only at the end.
I've read that Dawn has the Akumajo title back due to the pressure put on IGA by the japanese audience, and thus I can't really make an assessment of it, but if this is the case for real, then I can infer that IGA was going with the line of "No Demon Castle owned by Dracula is present, thus it's Kyassurubania again"..
We hypothesized this after looking at the Lords of Shadow's series names in japanese, and discovering that the first game is NOT called Akumajo Dracula in Japan, probably because there is no Dracula nor a castle of his in it. Then, Mirror of Fate is still not called Akumajo Dracula, but the name itself is mentioned in the game. Finally, the last game (Lords of Shadow 2) is called "Akumajo Dracula: Lords of Shadow 2". It's the only game to carry the "Akumajo" title because, as we hypothesized, Dracula is already fully established as the owner of a castle widely known as Dracula's Castle, and the game focuses heavily in their relationship.
So that's it; What we believe to be the reason behind the name change. If anyone can fill me in an "official reason" for it, I'd appreciate it very much!