a) I've changed brake lines before, It's not hard, the only real hard part is bleeding the lines to make sure that there isn't any air in them.
b) For regular street riding you won't notice a difference if you change the brake lines, pads you might depending on the pad, be careful though there are some pads out there that need to be warmed up before they are effective so don't go with to much of a race brake pad
c) The steel braided lines prevent the line from expanding therefore increasing the amount of pressure needed to stop, the expansion of the brake lines is caused by excessive heat that is transfered to the brake fluid, if you race or ride really hard on the street it might be worth it to you.
d) It is quite safe, if you know how. If you are doing just the pads its pretty easy to do. If you are going to go deeper into the brake system have a qualified tech give you a hand and look over your work.
The smart thing to do is to only change one brake and see how you did, this way you still have the other brake to rely on if you screwed up. After you finish do a test run, just go up and down your street when there is no traffic around, don't go faster than idle your first attempt at braking and go a little fast each attempt. If your brakes don't quit working after the first 10 or so back to back hard brakes then most likely you are good to go.
I would say that if your brakes are still in good shape leave them as is. When it comes time to replace the pads upgrade then. The stock ex brakes are pretty decent for a single rotor setup. Start out by changing the pads, if you still feel like you need more braking power (and you are riding on the street) then slow down you don't need to be going that fast.
I've been tought how to do it and it's not hard, but for the most part the brakes I leave to a qualified mechanic (aka my friend Randy) and I just go and help out (aka be his ***** while he does the work).
Good luck, and if you do it and don't feel comfortable take the bike to a qualified tech and have them look it over.