So I just watched most of all Dracula battles throughout the series, including all the non-canon ones. Prior to Rondo, when Dracula explicitly says, "Grant me power!" before the demon appears, we never actually witness Dracula turning into a demon.
In CV1, he gets decapitated, his body explodes, and then there's suddenly a demon who, to be fair, resembles a stubby, deformed Pazuzu.
In Vampire Killer, presumably his soul is transferred into a painting with a glowing crystal embedded in it -- not a demon.
In Haunted Castle, his soul flies off-screen and then he returns as a giant bust of himself. This was what happened with Medusa originally -- in CV3, she had a full body, then by CV1 she was simply a head. It could be argued we see her alternate between full-bodied and decapitated throughout the series because she's constantly regenerating her power between games: she gets a body, loses it, fights as a head while her body recovers, gets defeated as a head, then goes back to her body which has had time to recover. Back on track, Dracula isn't a demon here either.
In the first Gameboy games, Dracula just turns into a bat, not a demon. In the third game, after Dracula is defeated, the screen flashes and there is suddenly a demon there. We don't actually witness Dracula turning into a demon.
We don't see anything in CV2, presumably because Dracula was too weak.
CV3 arguably has the first "verifiable" transformation into a demon, as this time his head stays on the screen while the demon appears. However - and this is why I put quotes around it - multiple heads then appear. There are many multi-headed demons in classic lore, including Judeo-Christian angels, that even spew bile like the creature in the second phase of the battle. Additionally, the Devilman series, which had been around nearly two decades by the time CV3 came out, has the demon Jinmen, which absorbs the souls of its victims and displays their faces on its body. But I acquiesce, taking into consideration the Haunted Castle battle, this second form could very well just be a hideous "bust" of Dracula. The third battle, on the other hand, I still refuse to accept was Dracula. It's very clearly Pazuzu, inspired by The Exorcist. CV1's final demon arguably resembled Pazuzu, but this one unarguably is Pazuzu. And again, we never actually witness Dracula turning into Pazuzu, it just appears out of thin air. Then of course, there is also the whole matter of Dracula being a sorceror who can summon demons, and not explicitly a vampire or even someone with demonic powers (yet).
CV4 has Dracula turn into a swarm of bats, which burn up in the sun. There is no demonic transformation at all.
RoB is the first time Dracula explicitly turns into a demon. Or rather, technically, he asks some entity to grant him power, at which point he turns into a demon. Or does he? Just before he pleads for more power, we see a dark, shadowy presence converge on him. So then does he actually turn into the demon, or does the demon consume him? I argue that throughout the series, prior to Castlevania 64, what we actually see is Dracula being consumed by demons who perceive him as too weak after the hero defeats him. DoS for once actually shows Dracula (Soma) turning into this demonic entity, but is he actually turning into the demon, or is the demon breaking out of him? Regardless, DoS takes place long after the other games in the series, so it warrants a separate argument...
Bloodlines arguably shows Dracula turning into a demon, but that game takes place much, much later chronologically than the others as well, as it's technically a relatively modern story.
The fact of the matter is the primary antagonists of the series have been trying to harness the powers of demons. Dracula was arguably the best of the group, in spite of getting his butt handed to him repeatedly. Just because someone tries to harness demonic power, it doesn't mean they themselves don't get overtaken by the demons. A person typically summons a demon, attempts to consult with it, and then if the person is too weak, gets possessed by that demon or murdered by its influence. The manuals and some of the haphazardly written games lead us to believe Dracula had full control over the demons he summoned, but looking at his boss battles, it seems evident to me that somewhere along the way he messed up big-time and got possessed by a powerful demon. Not to diminish Dracula's spiritual power, I think at some point, after being resurrected a few times or utilizing his influence over the demons (and Death), Dracula certainly could have eventually turned himself into a demigod. But all the evidence I've seen from the games taking place prior to RoB suggest to me that he was simply an astute practitioner of dark arts who got consumed by the demons he tried to control.