Re-compositing

Very similar concept in FCP using Relink. Select the clip you want to relink, then FIle->Relink Files...->Original Media and choose the new file. There are some restrictions, like I believe it must match the frame rate and the audio sample rate and channels as the one it's replacing.

https://support.apple.com/guide/final-cut-pro/relink-clips-to-media-files-ver26f5c8c9/mac

You could try the Relink function in the File menu to point to a new file. The files have to be identical in length, audio channels, frame rate for this to work.

FCP has this "thing" when you apply effects to footage that alter its scale, even in the downward direction — which you would expect would sharpen the image — actually softens the image ("quality loss"). Using aspect ratio clips that are different from the project settings can also cause some softening when certain kinds of effects are applied. [There seem to be some issues where the "upscaling"/conforming algorithm gets "turned off".] If you're doing some edits like this, then put your (individual) clips (that need it) in a Compound Clip and apply your effects to the CC instead. [You can Command-C copy the clip, turn off all applied effects with the ☑Effects checkbox, apply the CC, then Command-Shift-V to paste attributes and choose which ones to apply — it will save you some time.]