「Assets/Resources/Prefabs」以下のUILabelに設定されたFontを一括で設定できるツールです。Editor拡張で作成。

using UnityEngine;
using UnityEditor;
using System.Collections;
using System.Collections.Generic;

public class FontReplace : EditorWindow 
{
	private UIFont font;

	[MenuItem("Tools/Replace Font")]
	public static void ReplaceFont() {
		EditorWindow.GetWindow(typeof(FontReplace)).Show();
	}	

	void OnGUI()
	{
		font = EditorGUILayout.ObjectField("UIFont", font, typeof(UIFont), true) as UIFont;
		if (font != null) {
			if (GUILayout.Button("Replace font in all assets")) {
				string title = "Replacing [" + font.name + "]";
				EachAsset (title,font);
			}
		}
	}

	private static void EachAsset(string _title, UIFont _font)
	{        
        	string[] sarchDir = { "Assets/Resources/Prefabs"};
		string[] guids = AssetDatabase.FindAssets("", sarchDir);
        
		for (int ii = 0; ii < guids.Length; ii++)
		{
			string guid = guids [ii];
			string guidPath = AssetDatabase.GUIDToAssetPath(guid);

			EditorUtility.DisplayProgressBar(_title, guidPath, (float)ii / (float)guids.Length);

			GameObject go = AssetDatabase.LoadAssetAtPath<GameObject>(guidPath);

			if(go != null)
			{
				List<UILabel> uiLabelList = go.GetComponentsInParentAndChildren <UILabel> ();
				for (int jj = 0; jj < uiLabelList.Count; jj++)
				{
					UILabel uiLabel = uiLabelList[jj];
					if (uiLabel != null)
					{
						uiLabel.bitmapFont = _font;
						EditorUtility.SetDirty(uiLabel);
					}
				}
				AssetDatabase.SaveAssets();
			}
		}
		EditorUtility.ClearProgressBar();
	}
}

public static class GameObjectExtension
{
	public static T GetComponentInParentAndChildren<T>(this GameObject gameObject) where T : UnityEngine.Component
	{
		if(gameObject.GetComponentInParent<T>() != null)
		{
			return gameObject.GetComponentInParent<T>();
		}

		if(gameObject.GetComponentInChildren<T>() != null)
		{
			return gameObject.GetComponentInChildren<T>();
		}
		return gameObject.GetComponent<T>();
	}

	public static List<T> GetComponentsInParentAndChildren<T>(this GameObject gameObject) where T : UnityEngine.Component
	{
		List<T> _list = new List<T> (gameObject.GetComponents<T> ());

		_list.AddRange (new List<T> (gameObject.GetComponentsInChildren<T> ()));
		_list.AddRange (new List<T> (gameObject.GetComponentsInParent<T> ()));

		return _list;
	}
}