I would happily accept Kheldian knockback being reduced to KnockDOWN on some of their attacks. But.
There are only a handful of characters in this game that can enjoy the satisfying level of flinging their enemies across the battlefield with big hard-striking attacks. It's also something many players push to remove from the game, entirely, due to the frustration it causes them.
To me, there is very little in this game that feels more dramatic than a permahastened perma-lightform Peacebringer throwing big and wild knockbacks into a group of enemies and watching them flail through the air, helplessly ragdolling, before the sheer power being used against them crushed them against a wall in a useless pile.
One of my most important characters was a Peacebringer. And without that raw sense of power, she would not have felt half as impressive.
And no. KnockUP would not replicate that feeling. If anything it'd look freaking weird. Yeah, Warshades do a lot of gravity style control stuff 'cause it's what they're meant to evoke. Peacebringers aren't their "Opposite", it's not a binary. And smashing you in the face with a big rising double-fisted overhead strike should not cause you to spontaneously leap up to smash your face to the ceiling.