Pope Francis made some recent comments in a letter, which are being summarized in headlines like, "Pope Francis tells atheists to abide by their own consciences."
Does the full context of the Pope's remarks represent a change in doctrine?
A change in doctrine, if it is possible at all, can only be expressed with a definitive statement ex cathedra. The Pope wrote in his letter
so no, it does not express any change in doctrine.
In the English translation, the Holy Father writes
That is a definitive statement of what God expects of mankind.
He goes on,
This is saying much the same thing as Benedict XVI did when he spoke about the use of condoms in combatting AIDS. Consideration of one's actions and the reasons for them is the first step on the road to faith.
In explaining that God's mercy has no limits, Pope Francis writes "if he who asks for mercy does so in contrition and with a sincere heart". That presupposes that the person asking for mercy is asking of a God he believes in [or who is he addressing?] and with sorrow for not having believed in the past. The Pope has not contradicted the tenet that belief in God is necessary; far from it. What he has done, though, in directing people to obey their conscience — their God-given sense of good and evil — is to point them in the direction of God.
Both Popes are encouraging that first step in searching out the truth.
Seek the truth; find it and repent; and you will enter heaven. This is consistent with the teaching of the Church.
I will quote the Council of Florence and let you draw your own conclusions.
Pope Eugene IV, Council of Florence, “Cantate Domino,” 1441,
Decrees of the Ecumenical Councils, Vol. 1, p. 578; Denzinger 714.