Rosh Hashana 17b quotes a Baraita
[...] If one repents in the meantime, between Rosh HaShana and Yom Kippur, he is forgiven; if he does not repent in the meantime, then even if he later brings as offerings all the rams of Nebaioth in the world, which are of the best quality, they do not forgive him in the Heavenly court. This implies that once one’s sentence has been decreed, even repentance cannot alter it.
(The Gemara then clarifies that this is referring to an individual, not a community)
However, isn't it commonly accepted that teshuva for a sin is effective anytime* - whether before Yom Kippur, or after it, or even many years after its commission?
*(even if aseres ymei teshuva are the most opportune time for it)