Can I replace the head casting on my YHS (Standard Series Yard Hydrant) with the more robust YHP (Premium Series Yard Hydrant) head casting?

NO!

The components are different and even if you were able to find a way to modify the components and make it work. you will be setting yourself up for serious problems down the road when replacement parts are required.

The selection of the proper parts required to repair your hydrant is determined by identification of the head casting logo or markings. This is true for all brands and models of yard hydrants on the market. Therefore, if the head casting were to be miss-matched, like if for example, a YHP head was put onto a YHS hydrant, then whoever ordered parts in the future would get shipped parts that simply will not fit or work.

Mismatching of Yard Hydrant parts should

NEVER be done EVER!