B. R. Ambedkar was a Dalit (Untouchable) rights activist, who in the 1930s was convinced that Hinduism was not the right religion for Dalits and it was one of the main reasons Dalits were put in deplorable conditions. In 1935 he met with Christian, Muslim (Ambedkar would later speak out against Islamic ideologies in India), Sikh and Buddhist religious leaders, and he was swayed to become a Buddhist. When Ambedkar publically converted to Buddhism it lead to a mass conversion of Dalits to Buddhism. In fact a lot Buddhists in India have ancestors who were once Dalits.
But let's say that Ambedkar doesn't convert to Buddhism. How different would India be had Ambedkar converted to Islam, Christianity or Sikhism?
Well, the big two ones would be Islam and Sikhism.
With Islam, which had a cultural presence in India and conflict with Hinduism, it would skew power dynamics more toward the Muslims and ultimately, it could be that the parition of India may happen differently as a result since there would be a shift in the dynamics. More of the lower classes may embrace Islam and lead to a classist struggle potentially.
With Sikhism, it is a similar case. It lacks the caste system and was historically formed from Hindu-Muslim interactions and schools of thought. A wide scale conversion of Sikhism would lead perhaps to more migration and focus on the Punjab region, the birthplace of Sikhism and perhaps Sikhism could rise up to become a third player in the power plays of the subcontinent. This could mean that besides India and Pakistan, we'd get a third area foe the sikhs (though given location, I reckon the Sikhs would get what would be Pakistan and how India would divided for Islam and HIndu follwoers is hard to say... maybe a north/south divide?)
Christianity is the big wild card, especially which brand of Christianity