Slightly embarrasing to admit though since my own research is in the sengoku era but I have started to wonder about one thing and anyone might have heard the reason.
The second Tokugwa Shogun, Tokugawa Hidetada, recieved the first kanji, hide, from Toyotomi Hideyoshi. Yet he did not change his name, a well established custom, even though Tokugawa attacked Toyotomi during Osaka sieges 1614-15.
Does anyone know why he received the kanji from Hideyoshi? The reason to why he did not change might be harder to find I guess.