First question, can you close one eye and touch your index finger to your nose?
There are two options for upgrading to ER9x. You can solder 6 wires to soldering pads in various spots around the tx, or you can buy a board that touches these soldering pads. Both methods involve using a program to 'flash' the new firmware onto the ATMEGA chip.
I had a go at soldering. I'm not the world's best solderer, but it was really easy and is about 1/10 of the cost. I didn't leave anything behind in the tx, just unsoldered the wires after.
Why should you do it? It's just better. More functions, easier mixing, nicer to navigate and use.