The creature loses all other creature types, but it retains its abilities. Wild Shape will overwrite any previous effects that set the creature's power and toughness to specific numbers. Effects that otherwise modify the target creature's power and toughness will still apply no matter when they took effect. The same is true for +1/+1 counters.

Both the target creature and the mode are chosen on announcement. You can't choose a different form if something changes in response to the spell.