The way the Asset Types are handled was changed in v6. They are really more of a template for the asset data now than they were in v5.
I've spoken to the programmer about this, and what she said is that during the design meeting, they discussed it both ways. Since the assets have their own individual fields, and 100 assets all with the same asset type could have 100 different manufacturers, the decision was made that editing the asset type would not automatically overwrite the individual assets' data. Instead, if you go to the asset list, select the assets that need to be updated by either holding Ctrl and clicking each, or click the first one and shift-clicking the last one, then click Mass Update. If the Edit Inherited Asset Type Data option is enabled, you can update the specific fields, or you can select the Asset Type again to update all of the fields.
A feature request has been created to either have a new option created or a check box added to refresh asset data when asset type is edited, and they will discuss this as a possible redesign for future patch versions.