Can a marginally competent person change the lock mechanism on their own? I imagine so, if you can work on your own guns the lock mechanism on a safe is not a long jump. I don't have any electronic mechanisms (Hell, two of mine are Navy surplus and over 60 years old or so) but I do service them myself every year. Clean them up and all, a little light oil. Just like a gun. One of the mechanisms I serviced years ago got sluggish and I brought it into a locksmith friend of mine. I used a lubricant where I shouldn't have and that gummed things up. Lesson learned.
Lock mechanisms are pricy. I have looked into newer safes with replacing the lock in mind and I wouldn't do it myself, better off replacing the safe with an older model that has an "analog" tumbler.
Oh, and another thing, on some tumblers you can change the combination yourself. I think I have a Yale that I can change the combo on. Be careful to have the new combo written down though so when you die your heirs can get it open.