OK, 我使用如下方案，好像实现了用户权限的迁移。不知道是否存在潜在的风险，如果有风险，还望有大佬给指出来。
1，导出旧tidb集群权限相关的表。
mysqldump -h node2 -P 4000 -u root -pRootPasswd mysql user db tables_priv columns_priv > old_user_priv.sql
2, 连接新的集群，导入上面导入的权限数据。
alpha@node2:~$ mysql -h node2 -P 4000 -u root -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 3827302424
Server version: 8.0.11-TiDB-v8.5.0 TiDB Server (Apache License 2.0) Community Edition, MySQL 8.0 compatible
Copyright (c) 2000, 2024, Oracle and/or its affiliates.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> use mysql;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> source old_user_priv.sql;
Query OK, 0 rows affected (0.01 sec)
Query OK, 0 rows affected (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
mysql>
3, 验证连接有效。