In order to change the type of a symbol, the tool must recognize, or infer, or be told explicitly the original type of that symbol.
The fixtype command is a way of explicitly telling the tool the original type of a symbol. As an aside, It is usually more effective to apply fixtype commands to function return types than to individual application variables because functions are used in type inference of many symbols.
The target attribute of the fixtype command is the original type, a vb or COM type of the symbol. It is not the desired .net type.
The desired .net type is typically specified in a modified copy of the IDF that defines the original COM type, by adding a migname attribute to the coclass.
Of course this only works if you are loading the modified IDF that contains both the original type and the IDF attribute. This is done with a reference command.
The FMStocks sample illustrates how to load custom IDFs for ASP site upgrades that include COM replacements.