Currently, we have a robust featureset allowing multiple Elements to have their Properties changed at once.
This is done by first multi-selecting Properties that have Properties in common, and then changing the value of their common Property.
This results in all Elements having their Property changed at once.

This mechanic does not however seem to function with changing the Easing value of multiple Keyframes at once, and it instead applies the easing only to the last-selected Keyframe in an array.

Repro Steps:

1) Create a new Studio Project featuring a MainPage.xaml
2) Introduce a Rectangle to the root Grid of the Page, set its dimensions to 512x512, and set its Render Transform Origin to Center-Center (0.5, 0.5).

3) Navigate to the Animation/Storyboards Panel, and create a new Storyboard Animation.
This will engage 'Recording Mode' on the Stage.

4) With 'Recording Mode' active, select the Rectangle introduced in Step 2), and in its Render Transform Properties, set its 'Scale X' to '2', and its 'Scale Y' to '2'.
This will result in keying two keyframes onto the Timeline.

5) Expand all expandeds within the Timeline, to reveal the 'ScaleX' and 'ScaleY' keyframes.

6) Using either Ctrl+Click, or clicking-and-dragging a marquee around the ScaleX and ScaleY keyframes, select both keyframes at once.
This will reveal the Keyframe's Property's in the Properties Panel, with in particular, a visible Interpolation graph.

7) With the Keyframes Properties open, select the right-most 'Easing' tab above the graph, and select any preset from the list.

8) Deselect both ScaleX and ScaleY Keyframes, then select each of them one by one, paying close attention to their Properties Panel contents.

Observe how only 1 of the 2 selected Keyframes had its Easing Mode changed.

Expected Results: Multi-Element Property value-changing also applies to changing the Interpolation method across many Keyframes at once.