Objection 1. It seems that fate is not unchangeable. For Boethius says (De Consol. iv): "As reasoning is to the intellect, as the begotten is to that which is, as time to eternity, as the circle to its centre; so is the fickle chain of fate to the unwavering simplicity of Providence."
Objection 2. Further, the Philosopher says (Topic. ii, 7): "If we be moved, what is in us is moved." But fate is a "disposition inherent to changeable things," as Boethius says (De Consol. iv). Therefore fate is changeable.
Objection 3. Further, if fate is unchangeable, what is subject to fate happens unchangeably and of necessity. But things ascribed to fate seem principally to be contingencies. Therefore there would be no contingencies in the world, but all things would happen of necessity.
On the contrary, Boethius says (De Consol. iv) that fate is an unchangeable disposition.
I answer that, The disposition of second causes which we call fate, can be considered in two ways: firstly, in regard to the second causes, which are thus disposed or ordered; secondly, in regard to the first principle, namely, God, by Whom they are ordered. Some, therefore, have held that the series itself or dispositions of causes is in itself necessary, so that all things would happen of necessity; for this reason that each effect has a cause, and given a cause the effect must follow of necessity. But this is false, as proved above (Question 115, Article 6).
Others, on the other hand, held that fate is changeable, even as dependent on Divine Providence. Wherefore the Egyptians said that fate could be changed by certain sacrifices, as Gregory of Nyssa says (Nemesius, De Homine). This too has been disproved above for the reason that it is repugnant to Divine Providence.
We must therefore say that fate, considered in regard to second causes, is changeable; but as subject to Divine Providence, it derives a certain unchangeableness, not of absolute but of conditional necessity. In this sense we say that this conditional is true and necessary: "If God foreknew that this would happen, it will happen." Wherefore Boethius, having said that the chain of fate is fickle, shortly afterwards adds--"which, since it is derived from an unchangeable Providence must also itself be unchangeable."
From this the answers to the objections are clear.
Source: Summa Theologica (New Advent)