HTC has a shiny new Sense UI version for its new flagship, but the 2013 flagships will get it too. A leaked table shows HTC plans to update (or not) its phones.
According to the table, the phones that will get Sense 6.0 are the 2013 HTC One family - One, One Dual Sim, One Max and One mini along with the HTC Butterfly S.
This will happen in the June-July time frame. Note that they already have Android 4.4 KitKat with Sense 5.5, this update will just update HTC's custom software.
The 2012 phones will not be getting KitKat or even Sense 5.5 according to the table. The KitKat/Sense 5.5 update for the Snapdragon-powered One XL is being evaluated. Even if that goes forward, the XL won't get Sense 6.0 though.
HTC's Desire 601 and Desire 700 should be getting KitKat/Sense 5.5 in April.
Note that this leak hasnt been confirmed by HTC so the timetable may have changed or be completely inaccurate.