I think changing a HG models DOES requires handfull of skills to make it into a MG level, but I ALSO think that making a MG/ PG even better (although it's near perfect) into something beyond what it's already is would be a greater challenge to Modelers.

As for me I stopped buying HG models and stick with MG, because I find them more challenging to built and the extended amount of time it takes to finish the product. =D