I’m working on a little sci-fi story. I have three parts written. The first part was written almost twenty years ago, the second was written five years ago, and what I have of the third was written yesterday. Obviously, my style and skill has changed much over the intervening years, and it shows! At what point, given the situation, does one forgo an edit of the older portions and simply begin anew?
I would say, although this is purely my opinion, that if your writing style in the older peices is obviously different then they need to be rewritten, as if the peice keeps changing in style- unless for plot, surrealist etc issues- then it would disorientate the reader. I had to do that with my novel, because i learnt so much about writing that it drastically changed my style. The other option, unless by changed you mean improved, would be to try and write your new segment in the same style as your old peices.
Even a rewrite wouldn't be completely starting over. Why not go through the older parts and highlight the phrases that you like and want to keep, and the plot points that you really want to bring out. Then when you revise, it's not that you're scrapping the original, but rather using the prototype to build a better finished product.
I have a short story that I wrote about 4 years ago. It's quite flawed, but I still like the basic plot points. I decided a couple days ago to start fresh. Everything except the basic storyline and the nature of the other principal character, is getting rebuilt from the ground up, beginning with a complete redefinition of the main character. Why did I decide to scrap it and start over? The main character was flat, and the setting she was in was uninteresting and cluttered. Changing those two things invalidated nearly everything I had written, even though the second most important character is changing very little. Rather that try to do a patchwork revision, I felt the new story would hold together better if I rewrote it on a clean slate. Even the second character will hopefully improve from starting clean, without referring back to the original draft.
I find myself in the same situation. I have gotten some good reviews on the Two Bullets Left* story I have posted on the site from Kaij and from Aurora Black(super-extra-ultra THANKS, you two.) They both pointed out the same flaws (using different words.) And after doing some of my own reviews on other works, I am very much in agreement with them. The stye of the first section now comes across as pretentious and artsie to me and just makes me cringe (blech!) The second portion doesn't make me hate it, but it needs work. The last portion just seems so much smoother and more streamlined. *If anyone wants to take apart the second section of the story, I would be eternally gratefull for the help! Short Story section, Science Fiction! . . .
in your case, i'd say now... it will end up being a patchwork, if you don't... keep all the various parts and permutations in mind and refer to them, as needed, but start from scratch and build a seamless work that will read smoothly, rather than trying to cobble together disparate bits from here and there...
It's tough to start over, it makes me feel like I'm betraying the original creative moment I was having at the time. But sometimes it's just what you have to do to get the story done right. I agree with InkDancer, just go over the whole thing today and get it to where you like it. Then it'll be consistent.
Those are huge gaps in time! I'd definitely rewrite from the beginning in that case. I find that if I let too much time pass between sections of a story--even as little as a year or so--then my style has changed so much that it's not worth continuing, at least without going over the entire thing and tweaking it to fit where my mind is right now. I can't stand the stuff I wrote just ten years ago and it is getting a major overhaul.