Hello,

It is currently not possible to subjugate realms that have aleady been subjugated (mainly by mordor). This however, makes subjugations by sauron permanent, as he can not die. I think it should be possible fight a war to change their subjugation status. Basically if you can win a subjugation war with their overlord defending them, it would make sense from both a gameplay and real life sense that you would now be their overlord.