That really is an ambiguous statement! And really it depends on the definition of fate and whether or not it can be changed normally. Then there's the decision as to what, exactly, they can change. Personally, I would see the powers of fate-changing as something that can be done indirectly, but not overtly. For example, imagine that fate is a river -- the Astrologian might not be able to change the river's destination, but they can throw rocks into it to alter the various things. Throw enough and maybe they can dam the flow outright.
In practical RP, this might be something like... using their powers to cast a protective shield around an ally to deflect a heart-seeking strike. Alternatively, they may be able to silence a mage and thereby potentially alter the outcome of a fight. If you wish to be more literal, you could even treat a spell as a 'luck buff'. Perhaps if you and your friends use /random for fighting, the spell could add 10% to the result. (Though I'd naturally say to make sure you only do this with friends or people who otherwise agree with that system -- I imagine altering rolls liek that unannounced may cause some upset!)
It's also probably best to ignore the question: "If someone changes fate, was it fate that their fate be changed?"