There was a dispute among various scholastics about whether the angelic will was 'immobile' -- that is, whether angelic choices could be undone. St. Thomas says yes, a number of others, like Scotus and Suarez, say no. I think St. Thomas is right, for a number of reasons both philosophical and theological; I think the 'no' side over-assimilates angels to human beings when in reality, despite the things we as rational beings have in common with them, angels would have to be very alien to us.
What is this immobility like? Whenever we get beyond a very general framework, there is always a speculative element in thinking about angels, but I think of the immobility of the angelic will as being like tree rings. Angels, being intellectual creatures, have free will; but every angelic choice is a choice of what to be. Angels don't undergo constant change the way we do; the change they do have is intellectual and volitional -- their only changes are learning and choosing. The scholastics expressed this by saying that we are temporal, but angels are aeviternal. Every change for an angel is a new age or era or epoch, an aevum. But all of this means that all angelic change is cumulative. Like a new ring in a growing tree, every angelic choice is, when added, just part of what the angel is, the angel's adding of a new aevum to its being. Wheels within wheels: every choice is a newly selected circle added to the circles that have already been selected -- a new way it is, added to the ways it already is. When an angel chooses, it chooses what it always will be. Every choice becomes part of every choice after; every choice is a choice of what kinds of choices the angel will ever make.
We have stories of angels sinning; we have none, of any value, of angels repenting their sins. When an angel falls, it has chosen to be fallen, and because it is a purely immortal being, its being fallen is contained in every other choice it will ever make.
This is not our own experience, but it is perhaps just imaginable that human beings could also have been this way. The story of the Garden gives us a sort of picture of how it could have happened. God puts Adam and Eve in the Garden of Eden, and allows them full rein, except that they cannot eat of the Tree of the Knowledge of Good and Evil in the center of the Garden, because if they do, they will die. The words are put very strongly; we could translate it as "Dying, you shall die" or "You shall die dyingly" or "You shall really die" or "You shall die to the utmost" or "You will absolutely die".
Then, we are told, Adam and Eve did eat of the fruit of the Tree of the Knowledge of Good and Evil, and the Lord God said, "Man has become like one of us, knowing good and evil." (Note the 'us'.) But lest man should put out his hand and eat of the Tree of Life and live forever, God drove man out of the Garden.
A punishment, to be sure. But also salvation. Fallen angels do not repent their sins; their choices are as immortal as they are. But we are mortal, and we are always dying, and our choices can die in our dying. That does not mean it is easy. But if we ate of the Tree of Life and became like the angels not just in knowledge but also in life, our sins would inevitaby be part of us forever, like tree rings.
It is a point that St. Thomas very, very occasionally notes, getting it ultimately from St. John Damascene: our salvation consists in God using our mortality against our sinfulness. We can repent because who we are in our choices can die and be buried. We can be saved, and raised to glory, because we can die with Christ and be raised to walk in newness of life. At least, we can until we either reach our utmost death or receive everlasting life, until we have the immobility of will that comes with spiritual death or with the completion of rebirth into undying glory. Then we will be like the angels.
