If you haven’t activated Shellephant’s ability, its power and toughness are 0/0.

If Shellephant has first strike, double strike, or triple strike, you can have it deal combat damage as a 3/3 and then change it to a 1/4 before normal combat damage happens. This will also work if the creature Shellephant is in combat with has last strike.

You can change Shellephant’s creature types at any time and while Shellephant is in any zone. It can be a Turtle, an Elephant, or both. “And/or” isn’t a creature type.