I know I'm late to the party on this one, but since people are still posting on this thread I would like to clarify a point here.
First, yes, if you replace the entire earpiece speaker assembly, you will lose Face ID. As Matt and Ian have pointed out, the flood illuminator that is part of the assembly is paired to the phone so putting one in that isn't paired will disable Face ID.
That being said, the earpiece speaker itself is ***not*** paired to the phone and can be replaced without losing Face ID. One caveat here is that it does require soldering; there are two small points on the flex cable that have to be unsoldered and resoldered to the the new speaker. If you have any soldering skills at all, it's an easy fix. I ran into that situation and wrote up a guide for it for the iPhone X, so I know whereof I speak.
So the short story is, with a minimal amount of soldering, the earpiece speaker can be replaced without losing Face ID. Here's my guide.
[guide|151215|iPhone X Earpiece Speaker Replacement Without Losing Face ID - iFixit Repair Guide|new_window=true]