The "Reimport with New Source" functionality is currently not functioning in the Editor. When selecting the option with a clearly edited Anim, it is not updated. There is no message in the Output Log stating whether it was imported or not either.
Importing the new Anim as a brand new animation works without issues, but reimporting with a new source files does not currently work.
This happens with single and multi-frame animations. I tried with multiple FBX exporting formats and with the Aim_CC, Aim_RU, and Crouch_to_Stand_Rifle_Hip anims.
Video below showing the repro.
Repro Rate: 3/3
Result: Observe that the original Anim is not overwritten and does not change.
Expected: Animation are overwritten when reimported with a New Source.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-142469 in the post.