Obviously this would have major historical consequences, seeing as it would make the Seleucids more acceptable rulers to the Persians.
If they had been able to assimilate as well as the Ptolemies in Egypt (or, perhaps better), perhaps the empire could have survived longer? After all, Antiochus was half Persian himself.
And, yes, I recognize that modern Zoroastrianism is not a convert religion, but it is recognized by scholars that early forms of the religion were much different than that practiced today--especially the Zurvanist branch that predominated at the time. It would not be too out of the question for Antiochus (who already possibly had a Zoroastrian mother) to convert, especially given the political significance of the act.
What are everyone's thoughts/opinions/ideas for a TL? IMO, we might see a serious contender to the Roman Empire's oriental expansionism (much as the Parthian empire in the second and third centuries of the common era), assuming that this act results in a more unified state.