Snakeform doesn't counter abilities that have already triggered or been activated. In particular, there is no way to cast this to stop a creature's "At the beginning of your upkeep" or "When this creature enters the battlefield" abilities from triggering.
If the affected creature gains an ability after Snakeform resolves, it will keep that ability.
If Snakeform affects a creature with changeling, the creature will lose its changeling ability, and will no longer be all creature types.
The power/toughness-setting effect overwrites other effects that set power and toughness only if those effects existed before this spell resolved. It will not overwrite effects that modify power or toughness (whether from a static ability, counters, or a resolved spell or ability), nor will it overwrite effects that set power and toughness which come into existence after this spell resolves. Effects that switch the creature's power and toughness are always applied after any other power or toughness changing effects, including this one, regardless of the order in which they are created.