It depends on the snake. If he's hyper, then wait. If he's laid back and doesn't normally get excited, change the water. However, it shouldn't hurt to leave everything alone for a couple of days either. I think it's really up to you, the snake won't care.
I've never heard of underfeeding causing any problems other than undergrowth. Severe underfeeding is called starvation and yes this will cause many issues.
Yes, you can overfeed. Regurg is the biggest risk. Impaction can also occur. If he's taking 3 rats at a time, regardless of how hungry he is, I think you need to go up a size on the rats. This is as long as the new prey food is the proper size.
Good Luck!
Jim Smith