If the majority of these great short par 4's are made great by their green sites, then wouldn't it stand that lengthening the hole any distance would not diminish the quality of the hole? The 10th at Rivera at 315 yards is a brilliant hole, as a par 5 stretched to 570 yards I'd imagine it would be just as great. Even if the tee shot is relatively benign, the strength of the second shot would set up the play off of the tee. If the 10th was stretched to a length over 340 yards, but shorter than 550 yards would it still be a great hole? If no one could drive to or near the green in one and the shot off of the tee was about positioning your approach would it be a dramatically worse hole? I could see If the hole played as a long par 4 or short par 5, the green would become greatly unfair as it would be unaccepting of a typical longer approach and would become unfair for the shorter hitters.
It's clear that a superior green site goes a long way in making a hole, there is precedence in taking a redan style par 3 green and using it when building a par 4. It appears that at the same time the tee shot from short risk/reward par 4's could be translated into par 5's. But the crux of the transition appears to be preserving the length and style of the approach when translating it to the new hole.