I'm not entirely sure anymore. I have updated my Walkthroughs before, which sometimes results in them going into new point boundaries. However, whilst they did initially go up, there were two periods where I mysteriously lost dozens of points due to incorrect calculations on the Stats page (it was actually my report to the Admins about these point losses which eventually resulted in the Stats page only being updated once a week to prevent incorrect calculations), and what submissions that I gained points for that were incorrectly calculated and later lost I'm not sure.
One I'm certain about, though, is that if you update a Walkthrough and it doesn't enter any new point boundaries, you won't get any points for it as you only receive how much it is worth when you initially submit it.
.