Does the HP of a Pokemon change after EV training it?
Will EV training the HP Stat of LV 100 pokemon change the HP of the Pokemon? (Increase in HP)?
Similarly, will, reseting the HP stat of the Pokemon decrease the HP?
Yes, As of Gen 5, stats are recalculated with respect to EVs after each battle, which means even for a level 100 Pokémon stats will go up/down if EVs go up/down.
Source: http://bulbapedia.bulbagarden.net/wiki/Effort_values