In Shul the rabbi changed the name of someone who was ill to somehow avert an evil decree. This seems to be a Kabbalistic custom. On a simple level, is it not trying to fool G-d? How does it tie in with Devarim 18 tomim tihyeh…
In Judaism, a name is not just a convenient way to call a person- it is the person's essence (Berachos 7b, see also Ohr Hachaim Beraishis 35:10). Hence, a person's name is intertwined with their mazal. Changing a person's name changes their mazal.
All the best