First, as Surtsey pointed out--Reddit is not a publisher. Posting something on an online forum does not generally equal publishing.
But there are things you can do to keep it from coming up. Delete the post. That's sure as heck possible on Reddit. Then you might want to do a google search on a different computer to see if the title even comes up in a search, or key phrases in the work. Might be empty, might not show up at all--in which case, it isn't a problem.
You can do that for sure. The fact that you showed an early version of the story online should not be a problem, however, the terms these days for publications vary. If they have anything posted on what they consider "previously published" or any sort of guidelines, that might be helpful.
Now, self-publishing something on a blog is a different matter than a discussion or critiquing post on a forum. That can depend on how well-read the blog is, and I know plenty of authors who put partials up on their blog for critique and feed-back. Different publishers have different standards as far as that's concerned.
Most of what I do isn't fiction, though I have sold the same story in different places. I haven't had experience with this specific thing, but again, posting something on an online forum to get opinions is not the same as publishing, unless they are following specific rules I don't know about.
Edit: For me, a publisher has never cared as long as it wasn't highly visible or searchable, once I informed them.
You're asking about editing to make it different, so I will treat this as though it were published in a print publication and how I would handle that.
Here's what I would look at:
- Are the plot points the same? Regardless of what you changed, if the plot can be described in short in the same way with the same characters, it might be considered the same story.
- If a single line, including the title is the same.
- If anyone reading can tell it's an adaptation, if they've read both.
- Be a great idea to change all the character names.
- If you change genre, as well, that will help
The rule is this, if another person had published the second version of this, would you be mad? Would you consider it plagiarism of your first draft?
These are the standards I would use as far as a print publication was concerned. But sometimes, if what you have is good enough, they don't care or consider it previously published.
My advice is to write to them and say something like this, depending on how strict their verbiage is on requirements: "I didn't consider this as previously published because the story has changed so much, but I wanted to make sure to tell you that very, very early draft of my story was up on Reddit for critique (it was taken down [several years ago] and is no longer searchable). All the characters names have even been changed--the only thing that remains recognizable from the original is the title, which we can change if you think it necessary." Then suggest a few alt titles.