FontReplacer.cs

using UnityEngine;
using UnityEngine.SceneManagement;
using UnityEngine.UI;
using UnityEditor;
using UnityEditor.SceneManagement;
using System.Collections.Generic;
using System.Linq;

public class FontReplacer : EditorWindow
{
    private Font font;

    [MenuItem("Custom/FontReplacer")]
    public static void OpenWindow()
    {
        EditorWindow.GetWindow(typeof(FontReplacer)).Show();
    }

    void OnGUI()
    {
        font = EditorGUILayout.ObjectField("Font", font, typeof(Font), true) as Font;
        if (font != null)
        {
            if (GUILayout.Button("Replace font in all scenes"))
            {
                ReplaceFont(font);
                Debug.Log("Replaced font in all scenes.");
            }
        }
    }

    static void ReplaceFont(Font font)
    {
        // 現在開いているシーン群
        var sceneCount = UnityEngine.SceneManagement.SceneManager.sceneCount;
        var scenePath2ActiveMap = new Dictionary<string, bool>();
        for (int i = 0; i < sceneCount; i++)
        {
            var scene = UnityEngine.SceneManagement.SceneManager.GetSceneAt(i);
            scenePath2ActiveMap[scene.path] = true;
        }

        // 全てのシーンを取得し開く
        var allScenePaths = AssetDatabase.FindAssets("t:Scene").Select(guid => AssetDatabase.GUIDToAssetPath(guid)).ToList();
        foreach (var path in allScenePaths)
        {
            UnityEditor.SceneManagement.EditorSceneManager.OpenScene(path, OpenSceneMode.Additive);
        }

        // Scene上に存在するTextComponentのフォントを置換する
        var textComponents = Resources.FindObjectsOfTypeAll(typeof(Text)) as Text[];
        foreach (var textComponent in textComponents)
        {
            textComponent.font = font;
        }

        foreach (var path in allScenePaths)
        {
            // シーンを保存
            var scene = UnityEngine.SceneManagement.SceneManager.GetSceneByPath(path);
            UnityEditor.SceneManagement.EditorSceneManager.SaveScene(scene);

            // 予め開いていたシーン群以外を閉じる
            if (!scenePath2ActiveMap.ContainsKey(path))
            {
                UnityEditor.SceneManagement.EditorSceneManager.CloseScene(scene, true);
            }
        }
    }
}