So far, I've seen NO indication that it will ever be time to replace the HV battery in a 2001 or later Prius. The batteries in the 2004/5 are better because internal resistance was reduced by better intercell connection. I think the 7.2V modules are compatible sizes/shapes, but I'm not sure of this. The 2004/5 batteries use fewer modules, thus have lower voltage, so you cannot directly swap full battery assemblies between the two models.
so far the most common question i get asked is
battery life and cost of replacement
it seems that everyone concentrates on that
and forget that all cars are made of parts that will some day fail
I have never heard of a single battery failure in any Classic or '04 Prius. A few Classics have racked up a quarter of a million miles. At this point it can safely be said that a battery failure any time during the life of the car would be a freak event. And with the 100,000-mile warranty (longer in some states) there will be used batteries available due to accidents before yours is out of warranty.
The battery is not a reasonable thing to be concerned about. On the other hand, the Prius has no tranny or clutch to wear out!