previa tyre question

No, you shouldn't change the tyre size from 215/55-17 to 215/45-17. Let's do some maths here. 215/55-17 has a circumference of 66.83 cm, while 215/45-17 has a circumference of 62.53 cm. There is a 6.88% difference in length. When you change tire size, the principle is to keep the difference within +/- 3%.
Your ride, fuel economy, and evening handling will be adversely affected.
RIDE: Because the side wall of the tyre is shorter, which means there's less air cushioning inside the tyre to absorb the bump.
FUEL ECONOMY: With the circumference numbers above, we figure out that it takes 1496 rotations for 215/55-17 to run for 1 km (or 100,000 cm); and it takes 1599 rotations to cover the same distance.
HANDLING: Your Previa's suspension geometry is not designed with a wheel and tyre package that's that much shorter than the factory spec.
SPEEDOMETER: There will be discrepancy. The speedometer shows 100 km/h, but actually you are only travelling at 93.5 km/h.