Depending on what type of equipment you have, it'll take ~2 hours to desolder switches, and a little bit longer for LEDs since the lead pins are smaller. Soldering will be ~1 hour. It's not too hard if you have past soldering experiences, just really tedious. Changing out springs and switches is even easier, but also requires desoldering/soldering.
If you ask a more specific question, you'll get a more specific answer. I could write out everything, but that will take too long. I also made a tutorial video on all the things you asked that has a bunch details and tips to do it.