Really because it does not actually sound discretionary?
That's the beauty of having a wonderful, logical forum mod. She has previously stated that nothing is set in stone because that would be dumb.
Obviously, the OP would not be altered in such a way to make the votes invalid, but if a logical(minorish) tweak came up in the discussion after the poll started there are no laws preventing alteration. This isn't court, this isn't even an official inno system, these forums are pretty much for players by players.
Edit: For example imagine a great idea where the thingy gave players KP based on the other thingy. A poll was created and everyone was liking it. Then someone pointed out that KP instants would actually be better than KP because of reasons. Everyone seemed to agree.
Should the OP be tweaked? Yeah, it should, and it would be allowed.
That would be "a substantive change", but a reasoning individual would recognize that the vast majority of voters would not alter their yes vote with the new tweak, and so there would be no harm in making the adjustment prior to forwarding it to the developers.