A Crusader converting to Islam or a Muslim lord converting to Christianity is much more probable. Each of those have a substantial enough population base to make a go of it. That kind of stuff did happen from time to time going either way, though only a case in 9th century Al-Andalus is coming to mind at the moment.i guess this post is null and void.
but one last thing..
what if say a high profile muslim warlord converted to either christianity/judaism and fought against the saracens
similar scenario just with a muslim instead of a christian converting