Auto Key animation - affect selected keyframe from a different frame in time

Hi guys,

I had an idea and tried to find something that could do this with no luck, so i think it doesnt exist yet.

Lets say you have some keyframed animation. Sometimes you want to see the result of changing some property in a specific keyframe and what it does after/before, but to change the property you need to go to the keyframe time, change it, and then scrub forward/back to see the result.

What if you could do this:

-You have a keyframe on 0 and another on 100.
-You want to see the effect of changing somthing from frame 0, but what happens on 50.
-Instead of going back to 0 to change it, you would change it directly on 50, BUT rather than creating a new key, it would change keyframe on 0.
You get instant feedback!

This could be done by making something that "IF" a keyframe is selected, then auto key does not create new frame, but rather changes the selected keyframe. If no keyframe is selected, it creates new keyframe as usual...

I know you can already use the curve editor and alter the values of a keyframe in order to see what it does over time, but many times you need to change many values so it makes it rather dificult and tedious...

What do you guys think?