It is not that hard to do it yourself. Just use jigsaw to cut the door, and install the glass after. We finished our door last year in 3 hours. Make sure you cut the right size!
Most people replace the insert by themselves, contractors don't do it coz they can't make a living by doing it.
We replaced our glass inserts(two inserts for two side by side doors) by ourselves. All you need is a screw driver.
It is much cheaper and less headache to replace the insert than replace the door, for example, you can't replace the door by yourself, you have to pay a contractor to do it for you and you have to paint the door after. Don't need jigsaw if the new insert has the same size as the old insert, just take the old one off and put the new one on.