Essentially, the new modification is to help clarify categorization of wind speed measurements based on miles per hour (mph), kilometers per hour (km/h) and knots. There is an inherent uncertainty when estimating wind speeds for tropical cyclones. Generally, wind speeds are rounded because it's unrealistic to identify exact wind speeds. So, these numbers are rounded to the nearest "5." For example, 132 mph would be rounded to 130 mph, while 137 knots would be rounded to 140 knots.
The new categories are shown in this graph from the NHC.
This provides a problem, specifically for Category 4 classification. Category 4 used to be defined as 131 to 155 mph (or 114 to 135 knots or 210 to 259 km/h). This has caused a problem in conversions. For example, if a reading is 115 knots, it is a Category 4 hurricane. However, when 115 knots is converted to mph (132.3 mph), and it is rounded to the nearest "5" (130 mph), the hurricane is then defined as a Category 3 hurricane.
The new scale will alleviate this issue. Category 1 (64 to 82 knots) and Category 2 (83 to 95 knots) will remain the same, but Category 3 (formerly 96 to 113 knots) will now be measured as 96 to 112 knots (or 111 to 129 mph or 178 to 208 km/h). Category 4 will now be identified as 113 to 136 knots (or 130 to 156 mph or 209 to 251 km/h), and Category 5 will be defined as 137 knots or higher (or 157 mph or higher or 252 km/h or higher).
This breakdown from the NHC shows the differences between the old scale and the new version.
Now, when knot-based measurements are used to categorize hurricanes, the conversion will not affect to which category the hurricane belongs. The NHC has stated that this new scale will not affect any previous categorizations of past hurricanes and has only been implemented to make classifications easier.
Information for this story as provided by the NHC.