You can't have the axis update its format. You can only set a single format for each axis.
One way to get what you're looking for though is to have two worksheets set up - one for home runs and one for batting average. Then when the user switches the value on your DisplayValue parameter, it will open up the corresponding worksheet and hide the other. It'll look like you're using a single chart but you'll actually be switching between two.
There are instructions on how to set that up here: