Can I change the lens on a Canon powershot Sz Is Digital camera to a stronger zoom?

Not exactly. If you buy the Conversion Lens adapter (about $30) and the Telephoto lens (about $100), you can boost the already hefty zoom on the S3 IS by 1.5 (making it from a 12x zoom to an 18x zoom). More than that is asking alot from a camera that isn't a dSLR with a very expensive telephoto lens on it.

The Canon Sx cameras have what is called a "fixed" lens, so you cannot take it off and replace it with a different lens.

However, Canon has add-on conversion lenses available for increased telephoto, wide angle or closeup. These screw on to a lens adapter, which screws on the front of the fixed lens.

Canon's conversion lenses are about $100 each and are very good quality. Cheap conversion lenses are available on E-bay, but are lower quality and unlikely to give you good pictures.

Good Luck