As I understand it, there exists within this site a standard such as allows a user to take a song they’ve submitted to the site and, provided it has been modified/enhanced/expanded upon enough, they can submit that new version of the song to something else.

My main query here is, what is the cutoff for that? How much extra song needs to be tacked onto an existing song and/or the original parts modified before it’s eligible to be a new entry? And beyond that, how much is socially acceptable regardless of technical allowance?

Currently an example I have on hand (that I may not even make use of under the current circumstances, quite frankly) is a song that was submitted to an OHB, and afterwards I expanded it such that it has at least double the original content and is three times the length of the original. Would that be the kind of thing one could submit to a major battle free of guilt, or would that be pushing it too far?

I've reused melodies and instruments but never modules, personally. That's just my experience though

ur example seems fine. like something that other people did before without issue.

I think it's hard to define specifcs here though. "if at least x % of your entry is new, it counts" can be different for every entry.
it's always a case-by-case thing. if you think you modified your submission enough, and that you spent a decent amount of time on it too, it is probably fine. that's at least how I would treat it

the more you try to define a fuzzy rule like this with specifics, the more people would try to brush up against the line. it's really just an honor system thing of feeling like you have put in enough new work on the track that you feel it's transformed into something else that stands on its own.

i'll offer a couple examples in case it helps - these are the two times i can recall doing this, in both cases taking something made in less than an hour (~20min on the first one iirc) and spending something like 15-20 hours making something else with that original ohb. the original is more like a prototype sketch or an initial idea than anything, and i put a lot more time into the pieces, so it feels justified to me.
43281 -> 47533
46152 -> 53248

I think it's just a case by case thing