im aware this wont happen because blizzard would not want to force players to race/faction change their characters. but were here on the lore forum so lets discuss it from a lore point of view. i also say all of this from the perspective of a horde main.
anyway to begin we can recall how the pandaren met the horde and alliance: an alliance airship with horde prisoners crashed into the turtle island and they had to work together to fix it. the alliance panda i think her name is aya wanted to wait around and figure out what to do but ji firepaw decided to blow the thing up and then they all had to quickly heal it.
i think they probably intended that to seem like both philosophies had a part to play but to me it came off more like ji was right because at least when he did something it worked out eventually. you have the horde and alliance healing it together so its not like one faction definitively did more to help. but i can see why at that point ji and some other pandaren would want to join the horde.
now lets consider everything the horde did after that:
discovered pandaria and set out to conquer it ("paint this new continent red")
began to racially purge the horde of all non-orc races including the pandaren who had joined them
picked up the heart of a creepy dead old god and dunked it into the extremely sacred vale lakes, corrupting them and destroying a rly nice looking neutral zone
imprisoned and tortured ji firepaw to the brink of death whereupon he was rescued by the combined alliance/horde rebellion force, most specifically the alliance panda monk girl
now do we think that after all of that ji firepaw thinks he made a good decision. does he feel that his life was not ruined by joining the horde. apparently he doesnt like them enough to show up for the dinner cutscene at the end of legion. and he hangs out all the time in legion in the monk hall with the alliance panda who presumably is having a great time because she wasnt conscripted into a war against her ancient homeland and then tortured by the person she swore allegiance to.
beyond that its becoming increasingly clear that even the horde pandas have basically nothing in common with the current horde's values. sylvanas's horde likes conquering people, enslaving val'kyr, creating weapons of mass destruction, and destroying cities. these are not congruent with the values we saw the pandaren express in the past. the tauren have a blood oath or w/e but the pandaren basically showed up because they thought it sounded more exciting than the alliance and history has proven they made a mistake
if you ask me it would be neat to have this happen even if no one actually cares about pandaren. it would give the alliance one of those fist pump moments or something to see their way of life chosen over the hordes. and it would make the horde re-examine themselves and how far they have fallen from what they were supposed to be. none of them care about ji firepaw not even baine he is having a great time at that dinner table where everyone else is either clearly disillusioned or outright plotting to start the next world war.
idk im just saying take some risks and spice up the lore blizzard when have we ever even seen someone switch factions.