Two issues you’ll be running into:
FCP gives a damn about the In and Out marks in your sequence, it’ll edit your Viewer’s In mark to the current cursor position, so you need to make sure you’re at the first frame of the clip you want to replace.
FCP doesn’t only replace the footage, it also replaces all your transformations and filters applied. If you just want to swap the material, you need to copy the clip before editing and paste the attrbutes (Option-V) after editing.