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He didn't "change" anything per se, but followed the current and made good use of the ongoing events so to speak. While he was the "Christian saint" emperor in Rome, at the same time, he erected the famous coloumn (recently being restorated) in his newly built New Rome (Konstantinopolis) on which a stutue of his figuring his as Sol Invictus (lost a few centuries later in an earthquake). He was playing for both sides; the Christian majority in the west and the pagan one in the east.

However, he did "convert" fully afterwards, holding the first ecumenic council in the new city etc. All those show his careful exploitation of the varying status quo.
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