Changing a DF from Logia to Mythical Zoan would be pointless, because it's already established that both Logia and Mythical Zoans are the 2 most powerful devil fruit types.
While Luffy's fruit was a Paramecia, got an upgrade after being a Mythical Zoan. With the exception of Earthquake Paramecia, every Paramecia is weaker than Logia and Mythical Zoan.
If Yami is a Mythical Zoan, and not a Logia, that wouldn't be an upgrade.
Enel's Logia being a Mythical Zoan as well wouldn't be an upgrade, so that would be pointless as well.
Plus, there are theories about Blackbeard will get a Zoan DF as his third DF power, if Yami is a Zoan, that would mean he would have 2 Zoans, and he wouldn't complete Logia - Paramecia - Zoan trio template.
While Luffy's fruit was a Paramecia, got an upgrade after being a Mythical Zoan. With the exception of Earthquake Paramecia, every Paramecia is weaker than Logia and Mythical Zoan.
If Yami is a Mythical Zoan, and not a Logia, that wouldn't be an upgrade.
Enel's Logia being a Mythical Zoan as well wouldn't be an upgrade, so that would be pointless as well.
Plus, there are theories about Blackbeard will get a Zoan DF as his third DF power, if Yami is a Zoan, that would mean he would have 2 Zoans, and he wouldn't complete Logia - Paramecia - Zoan trio template.