It’s either for the Bluetooth or AirPort or possibly WiFi. I don’t know which one. Given the possible applications it’s probably best to move it to the replacement case. Just be careful not to bend it when removing since I don’t know how delicate the internal construction is. Also, when installing it in the new case, make sure it’s stuck down well, maybe even add some new double side tape. Where it’s positioned, if it pops up it could block access to the optical drive.
Just finished replacing the top cover assembly. The biggest problem I had was re-inserting the ZIF connectors for the keyboard and backlight. There isn’t enough room for my fat fingers to line up the cable with the socket. The angled tweezers applied carefully (don’t want to puncture or rip the cable) and a little help from a spudger made it happen. When switching the cases check carefully for “missing” parts. Besides the trackpad, the upper case I got from iFixit lacked the center support bracket for the optical drive and logic board, the little screw bracket for the bottom case and the antenna stuck to the case behind the right speaker. The antenna is a little fiddly because it’s stickied to the case and there’s not much room to pry it out without bending and possibly damaging it.
All that said, it wasn’t a particularly difficult job, just labor intensive. And the wife’s reaction to getting her business computer back without buying a new one was worth it.
The only step you can skip is transferring the trackpad. To get to the keyboard you have to completely empty the top case. Plus, to replace the keyboard you have about a gazillion more screws holding the keyboard in place, The only advantage to replacing only the keyboard is cost
If you look closely, the four front screws (with the yellow circles) have a short unthreaded section under the head. The orange keyed screws are threaded all the way up.