If you replace it, only those you notify by yourself will know. The people who played it but moved on won't know. This is fine if your viewers are on a different platform (e.g. discord, patreon, etc.) and you funnel them to NG.
If you create a new one, then people will know about it from the Portal. In addition, it makes it more likely to receive awards because of the judging process. As bad as it sounds, fleshing out a game over time won't make it get the recognition it might deserve. This method is appropriate if there are enough differences between the two versions that people won't be confused about the two, or think that you're abusing the Portal/submission system. Examples where this is appropriate include sequels, massive overhauls (such that the old version becomes a "classic" version, etc.)
If you create a new one and delete the older one, then it's almost the same as creating a new one but the reviews of the people who played the older game will be deleted, in addition to the medals and other items. This is more of a nuclear option than the first two and really only done when the number of versions get out of hand.