Just wondered if anyone has beaten DB and then gone to do transcending the mundane quest. I feel like there should be different dialogue. Just wondering if anyone knows.

There is the complication that they already modified that quest to handle the vampire races for drawing blood in Dawnguard, so unless they managed to alter only the dialog... probably not.

I haven't actually collected all the blood types yet so I'm not sure if there's a bigger difference later on, but so far it's been very minor. For whatever reason, the dragonborn doesn't recognize HM's trademark tentacles when he appears and has to ask him who he is. However, HM does make a brief mention of Miraak's defeat and says he has been watching ever since.

Might have been a little difference but overall they only changed how he appeared. Instead of a purple abyss he was a mass of tentacles like in Dragonborn.

They did change in heavily.

Yes They did, the wretched abyss looks more like HM in the Dragonborn questline.

http://www.gamesas.com/topic/1429318-wretched-abyss-re-texture/