I thought コーヒーが sounds more natural than コーヒーは at the prior sentence, and the meaning doesn't change, right?

If I say コーヒーは, I'd like to change ポットには into ポットに. ポットにコーヒーはほとんど残っていない。sounds fine to me.

depends on the definition of "correct", if you read the complete rules it states：

“"Does this sentence have any spelling or grammar mistake? Does it sound weird?". ”

I'm not Trang, but when we did think about these rules, the main reason for this one was to avoid people changing correct AND natural sentences only because they suddently think an other version, with other tense or with an additional relative clause would be better, regardless if the sentence was already translated or not.

After, this was also to avoid people changing an old-style sentence or a quite "familiar" sentence, or at the opposite "high level of litteracy", only because they doesn't fit their way of speaking.

After if a sentence is grammaticaly correct, but would have never been said by a native, in any era or area, then feel free to modify it.

I'm kinda new adopting sentences here, I didn't know the background about making them sounds natural as CK mentioned.

So, should I undo the sentence and then add it as a new Japanese translation?

I would say, if this sentence is neither "old-fashion" (like your grand-father or so would have said) nor dialectal (well maybe it's easier for Japanese, as for example for English and French it can be canadian French/British English etc.), then you can keep your change :)

I see. I'll keep in mind that.