How to set a pose on the whole animation, with the movement of the previous one?

Hi. I have an animation I modified (used a mixamo pre made animation). but now i want to change only the legs. how can i change the whole keyframes of the animation without losing the idle movement?
do i need to redo the idle? or can i change that exact keyframe and set it to the whole animation without losing the idle?

image- the pose i have (wanna keep idle leg movemnt) - I just want to set the spine/knee straight (already applied to one keyframe)

this is the pose i want on the whole animation, with the movement of the previous one