First: There is no such thing as a HD antenna.. The existing antenna is just fine for HD, it is a TELEVISION antenna and from the point of view of the antenna, Digital and Analog are, well, the very same Radio Frequency, and that's all it cares about.
That said the older Winegard Sensar II's can benefit from a couple upgrades that give them greater range epically at UHF.
And there should be no need to get behind the TV either, nothing INSIDE the MH need be changed UNLESS... You are upgrading the indoor switch/power supply/splitter from either the Box of Many buttons or the standard wall plate with a switch, Which is always mounted where you can get at it so you can operate the switch, to a Sensar Pro (I recommend)
And neither of those should require you get behind the TV.