This is potentially much more complicated, and fiddly, than the guide suggests - depending on where you get the midframe from, there are a number of additional parts that need moving across to the new one - specifically the rubber seals from the buttons, the small microphone channel near the bottom mic, the plastic defuser for the LED next to the headphone socket and the front camera holder. Removing the lower speaker from the midframe is likely to break it, simply down to the glue being used to hold it in. It's worth making sure you have a replacement to hand as they're fairly cheap.

Additionally to re-emphasise, be very careful with removing the mother board, the vibration motor lead is very delicate and easy to tear. Do make sure it's completely freed up before trying to remove the board!