Bump, to save myself from having to create a second, similar thread.
According to many sources, the Paulicians of Anatolia (in the 860s and 870s lead by the warlord Chrysocheir from their capital of Tephrike) planned on converting Bulgaria to Paulicianism. They had had a significant presence in Thrace since the eighth century. Also, the turbulent religious environment in Bulgaria in the 860s - Frankish missionaries competed with both Papal and Byzantine monks, all with conflicting agendas, and Bulgarian pagans strove to preserve the old order - facilitated the spread of Paulicianism. The khan, Boris, also wavered between Latin and Greek Christianity several times in the 860s.
So what if the mission goes through, and c. 871 Boris receives a great Paulician mission at his capital of Pliska. Their doctrine is simpler than that of the Franks, Romans, or of the Byzantines. They promise that Boris will be allowed to control the church himself, as the Paulicians never developed a supreme authority figure like the pope or patriarch. All that is asked is that Boris adhere to the Paulician doctrine and aid efforts to spread it throughout Bulgaria.
One of the major benefits of course, is the complete liberation of Bulgaria from Byzantine influence, Boris's greatest desire. He would be able to increase his power as a ruler sanctioned by god rather than just a "first among equals". The boyars would be more content with this brand of Christianity, as their anti-Christian sentiment stemmed in OTL from a fear of Byzantine influence.
The Byzantine emperor of the time would undoubtedly launch an invasion of Bulgaria upon hearing the news - he did so in OTL in 864 when Boris converted to Catholicism. But the Bulgarians were more than a match for Byzantium. The only reason Michael III won in 864 was because Boris's army was away in the northwest, fighting Moravia. The full Bulgarian army could probably defeat the Byzantine forces and cement Paulicianism in Bulgaria.
Another major issue: the Byzantine offensive launched against Chrysocheir in 871, which lead to the fall of the Paulician state within ten years. A Paulician Bulgaria would probably ally with its brethren. Could the forces combine to destroy the Byzantine Empire and divide it up amongst themselves?