Authorship on a manuscript that has gone through significant changes I have an MS in a scientific field and have published several papers in the peer-reviewed literature in that field. Three and half years ago, while still working in that field, I began a side-project (unfunded, to be worked on in my spare time) and asked for assistance on that project from a faculty member at a nearby state university and a graduate student. They saw and commented on early drafts of that manuscript and indeed, the faculty member had the initial idea for the unfunded side project, though neither the faculty member nor the graduate student (now post-doc at a different university) did any data collection nor analysis. 

This manuscript has sat untouched since June of 2014. Neither the faculty member nor graduate student has asked for an update in the 3.5 years since.

I am no longer employed in the scientific field in which I received my MS, however I have continued to publish (albeit sporadically) in the peer-reviewed literature as an "independent researcher," mostly with extant data from previous projects. I recently picked up the above-described manuscript and began working on it again, changing significant portions of the methods, results, and text. Because of the changes I have made to the manuscript, I am considering dropping the faculty member and graduate student from authorship of the manuscript and including them in the acknowledgements.

Do I owe authorship to the faculty member and graduate student that were initially coauthors but have not contributed anything to the recent development of this manuscript? The manuscript isn't groundbreaking in the least (even in my field) but does advance the available knowledge.
I agree with the answer by @BrookeMcKnight.

I would add that, importantly, even if you have made substantial changes, it is your responsibility to reach out to your coauthors and afford them to opportunity to contribute further to the updated manuscript, otherwise you are somewhat stealing the authorship from them.

They may prefer to substantially edit what you have done, to provide minor comments, they may disagree entirely with publication and ask for major revisions or for their name to be dropped, etc - the key is that your responsibility is to give them the opportunity to participate and make those decisions. Their initial contributions did not expire over the 3.5 years the manuscript sat unworked on.