I really enjoyed this post.The actual Bant and Sultai philosophies do not emphasize UG as the core of their identities, so they may not be useful in understanding this particular planeswalker.
I personally think it's easiest to think about this particular planeswalker's three-color philosophies by breaking them down to their two-color components.
The planeswalker is primarily concerned with progress, a UG value.
As a Bant planeswalker, they probably valued order (UW) and community (GW) as a means of achieving progress. They might frame progress in terms of enlightenment or the development of society.
However, as a Sultai planeswalker, they probably turned instead to forbidden knowledge (UB) and exploiting the cycle of life and death (GB).
GW's focus on community has adverse effects on evolution, as GW opposes individuality. Meanwhile, UW's focus on order can prevent adaptability due to its rigidness. While society becomes more sophisticated over time, no new ideas can develop.
As the planeswalker values progress most of all, they may have become disillusioned with either of these white-aligned color combinations, believing that the philosophy they fostered in the name of progress is stagnant. They instead turn to GB and UB.
GB is the color of life and death, and of survival. They possess the raw drive to adapt and overcome. While they may not be as collectively powerful, they can overcome a threat that GW could not by virtue of the most resilient and well-adapted members surviving.
UB is the color of forbidden knowledge. Unlike UW, they do not restrict themselves to what is acceptable - they are fiercely ambitious, and will do anything for the sake of power.
As such, were I designing such a character's change in philosophy, I would probably frame their views regarding progress shifting to value a more individualistic desire to overcome and adapt, with a greater focus on the grittier aspects of progress, such as survival.