I don't think the ear piece is glued to the faceplate. It may appear that it's so, but it shouldn't be. There may be a foam guard/noise/vibration piece around the earphone, and that may indeed be attached to the faceplate. If it is, you are going to have to very, very gently remove that and put it on the new faceplate.
Also, there may or may not be a another piece of foam around the speaker area, once again, that should be transfered over to the new faceplate.
One other thing I found out about aftermarket faceplate's for LG phones is where the 3 line/holes are for the speakers, they have made them very small. A lot smaller than the factory faceplate. I ended up taking my faceplate back off and taking a very small drill bit and drilling them out. It helped tremendously.