Based on this interesting answer from u/frogbrooks. Like many people, I‘ve always assumed that Islam was always interested in converting other cultures and people. So, yeah when did this transition happen and why did it happen?
Islam has always been a proselytizing religion. It literally pervades every Islamic text, all throughout the Qur'an, the many different books of hadith, and the other various religious literature including exegesis and jurisprudence. If you can clarify what makes you think it wasn't a proselytizing religion, perhaps I can give an answer that makes sense.