Maybe not....
That is the VHF element for the Batwing, the UHF element is inside the case. So if you normally travel where there are no VHF stations you wont notice the difference. As far as replacement, yes, you can replace the head only. There are really only two choices: the current version of the batwing that is a better performer than what you currently have, or the Jack that is slightly less in UHF performance than whats on the roof now, but its VHF performance is significantly less. Just depends on what you can live with in the places you go.
BTW- its just an antenna, and that has nothing to do with HD