@A-tan my theory on this
i was trying to figure out the condition that allows demons to conquer the sun here's what i came up with:
1- the first demon to conquer the sun was nezuko and she shares the same blood as tanjiro so what she can do tanjiro can do it too( at least for the sake of this theory).
2- tanjiro can use the breath of the sun ( and there were only 2 breath of the sun users so far) so i guess if nezuko tried learning the breath of the sun she might be able to use it too.
3- also nezuko never tried conquering the sun before so she might have been able to do it from the beginning but never tried it because she never knew it was a thing.
so long story short, if a breath of the sun user (or a potential user) turned into a demon they can conquer the sun.
at least that's what i came up with trying to figure out the reason. it might be wrong but who knows it might be true.