db={"geoJson":[{"coordinates":"61.11736,-149.86379"},{"coordinates":",-149.86379"}]}

db.geoJson.update({},[{"$addFields":{"coordinates":{type:"Point",coordinates:{"$map":{"input":{"$reverseArray":{"$split":["$coordinates",","]}},"as":"c","in":{"$convert":{"input":"$$c","to":"double","onError":"","onNull":""}}}}}}}],{multi:true})