Changing individual position values in a series of keyframes

[Todd Kopriva]

You can select all of the keyframes and drag the x or y values in the Timeline panel. (See this page.)

Also, you can separate Position into its components if you’d rather work that way.

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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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[Greg Neumayer]

You could link the x or y to another value such as an expression slider. I do this sometimes to attenuate the overall effect of my keyframes. For example, I had a flickering light that I decided I wanted to fade up, so I linked that light level (which contained many keyframes) to an expression slider, and keyframed that slider with a start value end value keyframe.

Antifreeze Design
https://www.antifreezedesign.com