They should fit,
235 is the width of the tread in mm's, 45 is the sidewall height expressed as a percentage of the width (ie: 45% of 235 = 105.75mm sidewall height). The 235/45's will only differ from your 235/55's in the overall height of the tire, 45's will be shorter. A 10% difference comes out to ~23.5 mm difference in sidewall height, or twice that in overall height of the tire. As long as the tires are designed to fit your rim diameter, 17, 18, or 20", there should be problem, though going to a different height tire will throw off your speedometer some.