Waterfall_13
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Perhaps the game requires 2 (or more) "ultra" upgrades to be visible in the castle?
I tested it with the Phoenix, adding it as the second unit.
It didn't work; they don't upgrade, but the Phoenixes do occupy 1 slot.
This isn't what I need, but these changes led me to compare the current Death Knight file I created with the Phoenix file.
The difference was that the Phoenix's base creature is UNKNOWN, while the Death Knight's is an Angel, because the Death Knight file was replaced with an Archangel inside and then further edited.
By removing the Angel as the base creature and setting it to "Unknown," I was able to achieve this:
At the same time, Archangels can be normally upgraded to Seraphim for free, and Seraphim can be upgraded to Archangels (in my mod, upgrade = 0).
There is, of course, a visual bug in that the upgrade cost is displayed, even though it is actually free.
But in reality, the upgrade doesn't deduct any money because the cost for both is the same.
However, upgrading to Higher Archangels does deduct money as it should.
You could say this is 90% successful! =)
The only downside is these two bugs:
(Firstly, upgrading the Dweller to the third level will prevent us from recruiting regular Angels, which could be a problem in certain situations.)
(Secondly, when upgrading, it says that the regrade will consume resources, but it doesn't in reality. It's not critical, but it's unpleasant.)
Overall verdict: not perfect, but EXCELLENT! =)