True and I have done so on a number of occasions. When I revise a posted chapter, a new chapter is uploaded to replace the existing chapter. The stories completion status is not affected by that revision, which is what Docholladay indicated.
Morgan did it for years, literally five or six years in a row. I was told that he/she was adding chapters, but I don't know.
I do know that author, every year., was choosing a story under the old voting system (that favored old stories) slightly modifying it, and getting a new "completed on" date to get it on that year's list of top stories. All of the stories were completed many years before.
I even predicted which story he/she would "modify" the next year. I think "Kathy" was the last one I predicted. It now has a completion date of 7/2015 though it was originally completed in 2003 or 2004.
Things have changed, scoring is more fair across time and the point of doing this has faded, but it still may be possible to change a completion date without reposting the whole story.
**--FULL DISCLOSURE--** One of my stories has been completely reposted. It was contractual, I could not post it anywhere else as long as it was for sale on that site (stupid move). Rebecca Danced has a very marginally lower score now than when it was originally posted. (It's within .02 (lower now than then). I say that because earlier I was accused of reposting to raise the score (only fans would read/score it). I get that, and it kind of makes sense, but I thought I could sell it.