Do not change the water 100%. When a fish is introduced to the tank, or in this case bowl, bacteria are needed to stave off scale diseases. While a 90% water change can be suitable, never do all the water at once.
When you initially set up a new tank, placing one fish to create a healthy bacteria base is the recommended procedure. Since you already have a good bacteria start, keeping some of the water (as I mentioned, about 10%) is a good idea. That way, when you set up a tank for them, you transfer the entire bowl of water they're in now, to the tank. This moves the bacteria over and keeps the "slime coat" on the fish. If you choose not to do this, then you should buy a water additive to aide in the production of a slime coat for the fish. It will also assist in keeping the stress from changing from bowl to tank at a minimum.
I'm firm - but fair. Mind the rules and enjoy your stay.
According to the Humane Society of the United States:
There are an estimated 3-4 million dogs and cats euthanized each year in the US alone! PLEASE - spay and/or neuter your pets!