It wasn't too bad, if you've soldered a speedball kit, it's no smaller than that, it's just getting into tight spaces and having something (or someone, like my wife) to hold the pieces while you solder. I've done it twice, the first time I had my wife help me by holding it, last night I did a second pair and used a vice and that worked too. I've attached some pictures that I took before I did the upgrade. The biggest thing is just matching it up before and after.
I've done this mod to the two pairs of DT770's that I have. The first pair looked like the pictures both before and after, but with the second pair, the 600 ohm drivers had lugs, so it was a bit easier. (I didn't take pictures of them).
If you just keep track of which wires match up, it's not too bad.
In the pictures, the three wires (gold, red and blue) come from the cord and the the solid red/white ones go to the other side.
When it comes to taking them apart, there's several Youtube videos on how to do that, I think I used this one:
