Revisionism

Recently, I've been rewriting books I wrote before and a few things occurred to me. "Sunfire" is heavily rewritten, for example, but that was for a new publisher (Ellora's Cave) who had different requirements to the book's first publisher. I really thought the book was improved by the rewrite. But there wasn't much time before the original and the rewrite.
I'm currently going through edits for "Devonshire," the second book I had published and I'm finding myself less inclined to make any big changes.
Why? Because I wrote it long enough ago for me to have been a different writer then. I did things in that book I wouldn't do today, but that doesn't necessarily mean I'm better now - just different. I know what I was trying for when I wrote it, and it isn't always what I go for today. My concerns have changed, my writing style has changed.
And I revised "The Chemistry of Evil" ready to present it to publishers. I was happy to revise that because it is part of a series (Dept 57) that I'm still writing today. It gives me a chance to check the continuity. But I deliberately held back on heavy rewriting. I added a scene that I thought improved the story, but it's only a little one, and kept other things I wasn't entirely sure about, but I was then.
So where does revisionism stop and improving start?