differrence between HPS versions and JT campaign versions?

[Jagger]

Reading some threads, apparently I can upgrade my old HPS games to the new John Tiller campaign versions.  

So I am wondering what are the major differences between the HPS games and the John Tiller campaign versions?  Are there any significant game mechanic changes?  Better graphics?  Better interface?  Are the campaign games a significant evolution from the HPS games?

Thanks all.