Q. Under what conditions, if any, can a writer approach a particular agent more than once with the same project? How much of the original concept, platform, etc., has to change to warrant a second query?
A: It would have to be a substantial change in order to warrant another attempt. I see writers do this sometimes. They will give the work a serious overhaul and a new title then resubmit to agents. There's no guarantee if this will work but you are free to try.
Perhaps if the agent rejected it and gave you a very specific reason ("the ending doesn’t work" or "point of view confusion early on"), you could re-submit if the weak aspects were overhauled.
Speaking broadly, resubmitting in itself is a crap shoot because some agents will not look at anything they’ve ever rejected. Some have personally told me this. Others are more willing to give projects a second look.
Thank you to Allen Insurance for this cool photo.