When did Jesus change his view on preaching to Gentiles?

The field of harvest for Jesus was Israel, and concerning the law of harvest:

   Le 23:22 And when ye reap the harvest of your land, thou shalt not make clean riddance of the corners of thy field when thou reapest, neither shalt thou gather any gleaning of thy harvest: thou shalt leave them unto the poor, and to the stranger: I [am] the LORD your God.

There was a circumcision of the field. Inside the circle represents the Jews, and outside the gentiles.

The woman's clever response showed she had a knowledge of the law. Even the gleanings inside the circumcision were available for the poor and sojourners. And Jesus was compelled, in fulfilling the law, to answer her request.

In both cases, the harvest of the corners and of the gleanings came after the harvest of those in the circumcision.

His primary mission of harvest has to be of the Jews until after the cross. Paul also first went to the Jews both by Jesus's example and by his understanding of the 'dinner theater' aspect of the shadow of the law.

His 'other sheep'... those in the four 'corners' of the world are the gentiles who's harvest has to wait for the primary harvest to be completed.

Side Note: The woman had received the same kind of revelation as Peter in acknowledging Jesus as the Messiah, Son of God, since she identified him as fulfilling the 'shadow of the law'. Such revelation is obtained by faith.