DISCLAIMER; While I admit being critical of Islam and not very patient with some traditionalists let's say, this thread is NOT Islam bashing intended.
There is some clichés and tropes in alternate history. One seems to be - due to the Shariah rules on apostates... - that once muslim, always muslim. that is it, a culture, a nation, minority etc who converts to Islam will remains muslims forever (with perhaps the unique Albanian case, and even then, its more like secularisation and rise of atheism, who was also quite... condoned).
So, here is a simple challenge.
Have a culture... nation (ethnocultural sense).. region.. minority.. etc, yadda yadda, who convert en masse from Islam, to another religion (albanian cases fine).
CATCH; it have to not be forced upside down, albeit the people agreeing with the top-leaders is fine. It have to be agreed and willed, or just happens as if.
By example, maybe something like the 'Dalits Buddhism' movement, who pushed in India for the conversion of Hindus to Buddhism to "escape the castes". Or a good group of albanians converting back to Christianism to get western help; maybe callign them 'Skanderbergists(?)'.