Use pt-show-grants utility to generate a "logical dump" of your accounts and grants. You can do the same thing manually if you just run show grants for for each user, but this percona-toolkit utility will make it even easier.
Unless you have special configuration or authentication plugins, this should mostly work (unlike trying to dump and restore all user and priv tables physically). However, in some cases (in compatible permissions, roles, plugins), you may want to review after sending the sql for errors or missed grants. Also check for configurations that may make the loading failing, like temporarily disabling NO_AUTO_USER_CREATE and the default authentication plugin (native, not sha...).
I recently ported MariaDB 10.1 grants to MySQL 8 and it required some iterations, such as different names for the unix socket authentication plugin, the default authentication plugin (vs native) and the usage of VIA vs WITH. Mostly due to the default configurations, something you will have to work in anyway for the migration.
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