I have written a paper for an IEEE Transactions journal. But now I have several improvements to the original paper. Can I publish the paper again with a slightly different approach? The introduction, etc., would be almost the same and hence I may retain and refer to the already published paper. But the implementation is different. How shall I proceed?
If the improvements are sufficient to constitute a new paper, then write that paper, referencing the already published paper (but not repeating it) and explaining the improvements. If the improvements are not enough for a new paper, then I'd suggest preparing a new version of the published paper, incorporating the improvements, and posting it on the arxiv (or in some other highly visible place on the web).
I would recommend putting the new material in a tech report, and publishing it alongside your original paper on your website. This establishes prior art and makes the improvements available to other researchers, while also avoiding two ethically dubious alternatives: trying to publish the revision as a new paper, despite it being incremental over your original version; and substantially updating a peer-reviewed paper without the reviewers evaluating the changes.