I have been looking through my old drawings, from a more than a couple of years ago, and faults which I could not see at the time leap out at me. Most look under-worked, and too light in tone, owing to my phobia of pencils softer than HB, which I've found very prone to smudging in my hands. I made a mini-mahlstick to counter this problem not long ago. I've been very tempted to rework my older pieces and bring them up to something like my present standard. This I've started to do on such drawings as this:
White Ribbon by ~dashinvaine on deviantART
Which now looks like this:
Black Ribbon by ~dashinvaine on deviantART
Whether this activity is a good idea I don't know. It's alright when it involves minor changes, the correction of missed faults and general finishing off, but there probably comes a point after which it would be better to start again. This is especially the case when one is doing things that change the character of the original picture. There is also the consideration that it might be better to leave old works as they were in order to have a record of your progress!