I wrote a paper that contains a fairly long and intricate proof. Now I have a new paper I am working on that extends this proof to a slightly different setting. Even though the overall setting is only slightly different, in my opinion this small difference is still quite noteworthy. For example, there are a number of other papers which use the result from the previous paper, and one can almost automatically get improved parameters for these applications.
The overall outline of the new proof is almost identical to the previous one --- it consists of about 5-6 pages with ~10 definitions/propositions, which occur in the exact same order in the two papers.
However, the proofs are subtly different --- I would say that the difference are far too subtle to merely hand-wave away with something like "We can show using a similar proof that ...." These differences are not confined to a single proposition either, they are spread out across the entire proof.
So, I am faced with a few options which seem less than appealing:
Copy the old paper nearly verbatim, with new proofs in the appropriate places. This has the advantage that the new paper will be mostly self-contained and readable. But the downside is that the is massive (self) plagiarism --- I am forced to copy pages of text without block quotes or specific attribution. (I of course state in my new paper that I am heavily copying from my old one, so there is some attribution)
Try to rewrite the old proof in a basically different way. This avoids the plagiarism problem. However, this will definitely not help the reader trying to compare the new result with the old one. It also seems perverse that a proof which was carefully laid out and understandable has to be restructured for no good reason.
Just mention the differences between the new paper and the old paper. But this will be almost unreadable unless the reader has a copy of the old paper and compares the two papers line by line. I would be forced to say something like "as was shown in Proposition 6.13 of [old paper], we have the following.... We then apply Proposition 6.14 of [old paper] and ...."
I am inclined to go with option (1). (If I was not the author of the old paper, it would make me much more queasy). Are there any good guidelines or suggestions for this situation?