Dynamic keyframe value using expression?

Hello,  sorry for being this late (again). But I had the same problem, and thanks to @chrish18591230  I gave it a different try that worked for me. Here it is :

prop = thisProperty;
firstValue = prop.key(1).value;
lastValue = prop.key(2).value;

newValue = 500;//this value is to be changed for a controller or whatever

linear(value, firstValue, lastValue, firstValue, newValue);

If you link the controller to the newValue, you will be able to have what you want. 

My apologies for posting on a thread that's been inactive for a year, but I was facing this exact same problem in the middle of a big project with a short timeline. Now that the project is over and I've had some time to think about it without a deadline hanging over my head, I think I've solved the problem and wanted to add my solution here in case it helps anyone in the future.

//reference to the property you've animated
prop = thisProperty
//reference to the first keyframe of the property you've animated
firstKey = prop.key(1).time;
//reference to the last keyframe of the property you've animated
lastKey = prop.key(prop.numKeys).time;
//the total distance traveled between your two keyframes
distanceTotal = prop.valueAtTime(lastKey) - prop.valueAtTime(firstKey);
//the distanced that's already been traveled on any given frame
distanceCurrent = prop.valueAtTime(lastKey) - prop.valueAtTime(time);
//the percentage of the total animation that's been completed on a given frame
percentage = 1 - distanceCurrent/distanceTotal;
//reference to a slider on a control layer that is used to modify the value of our
//final keyframe
offset = thisComp.layer("controls").effect("property offset")("Slider");
//sets the value of the property to be it's current value plus a percentage of the value
//we set with our slider
value + offset*percentage;

In my example, I was modifying the X Position after having separated dimensions, but this could easily be adapted to any property by just changing what property is being referenced by pos. It works with both positive and negative numbers entered on the slider, allowing you to either increase or decrease the animation, and it handles animations with more than just two keyframes, distributing the offset correctly across the whole animation.

The only limitation is that the slider is used to offset the position of your final keyframe, not to set an absolute value for it, but I would think that for the purposes of adjusting templates, that's how you would want it to work anyway.

Hopefully this will be useful to someone!

*EDIT* I adjusted my expression to just reference the property it's on, so now you can just paste it onto any property you want to adjust and all you'd have to change is relinking offset to your slider control.

I'm thinking a solution would be as simple as this in the expression window:

No, it isn't. Keyframe values are read-only to begin with. Any expression can only overwrite/ modify the actual values, not persistently set them in any form. It's inherent in how AE evaluates property streams:

Property base value --> Keyframe value --> Expression result

Furthermore, retaining custom interpolation curves can only be done by referencing an existing curve using valueAtTime() and manipulating the actual time that is used for sampling the value. So if you wanted to reference your existing interpolation, it would still be something like

thisLayer.transform.position.valueAtTime(myTime)[0];

for the X position for instance. myTime could then be defined as your retimed original animation:

myTime=linear(time, key(1).time,key(2).time,newStartTime,newEndTime);

In any case, you need to adapt your thinking. It's more complex than just a single line bit of code.

Mylenium