Many recent cars and stereos use a standard ISO wiring connector. If the 2003 stereo has done so, you should be able to fit it into the earlier car quite easily. If it is a custom Honda connector then it depends on whether they have changed the design between the cars - I wouldn't have thought they would do however.
You say the radios "did not always occur on the other model", which implies to me that some of the radios were common across both years. If this is the case, then I would expect the connectors to be the same, as they must be the same across all stereos in a given model range unless adapters are used...
The only reason I can think that they would be incompatible is if the newer stereo has additional features that require extra connections that the old one didn't. I can't imagine big car manufacturers spend money re-designing bits that hardly anyone will ever see unless they have to!