It would be a massive power boost that could skip decades of human-time training, but can Demon Slayers become demons?
Yes, Demon Slayers can become demons. In multiple instances, Demon Slayers are tempted by evil to transition into demons as a means to increase their physical strength and fighting capabilities.
Some slayers like Rengoku reject the idea of becoming a demon, but others go along with the idea.
Demon Slayers are given the opportunity to become demons
There are multiple instances where Demon Slayers have the opportunity to become their enemies. A key example is when Akaza, an Upper Rank demon, propositions Rengoku to join him.
Rengoku refuses to become a demon, stating that it would go against his humanity. The Flame Hashira believes human-life has inherent value and nobility precisely because your time on Earth is limited. Growing old and passing away, two “faults” that Akaza dislikes, are a natural part of the human experience that turning into a demon would squander.
Becoming a demon comes with the added perk of living eternally (so long as no one slays you). That’s another improvement that Akaza informs Rengoku about, but the Flame Hashira remains steadfast in his decision to pass away as a human.
This situation actually works both ways, as demons can become Demon Slayers. Nezuko becomes an hononary Demon Slayer, and Genya is an official member.
In chapter #143, the final battle between Demon Slayers and evil has commenced. Zenitsu, now confident and wide awake, breaks off from the group in order to satisfy some unfinished personal business.
Approaching a room, Zenitsu says “You’re there, right? Come on out. I know you’re there.”
A demon’s hand slinks from around the corner of the door. In a broken and distorted voice, Zenitsu hears someone claim that’s no way to speak to a senior.
Then we see it. Kaigaku, a Demon Slayer that Zenitsu trains with under the former Thunder Hashira, has become a demon. “Now that you’ve become a demon,” Zenitsu says, “I no longer think of you as my senior.”
So that’s the answer. Demon Slayers can indeed become demons. This transition usually happens because the Demon Slayer wants to quickly obtain new power and fighting prowess at the sacrifice of their own humanity.