Does HDTV transcend NTSC, or are there PAL and SECAM variations as well (or would there need to be if regions employing those standards wanted to upgrade?)
HDTV transcends the whole NTSC/PAL/SECAM debacle. There are digital resolutions that are PAL derived (like 576p), but HDTV stands apart from the old competing analog standards.
There are two (ETA: three - a Japanese one too?) major HDTV standards, but they both support a range of frame rates and resolutions, and interlaced and non-interlaced video. So converting between the two is more feasible and can involve no loss of picture quality. Whereas converting from 60 fields per second, 480 line video (used in NTSC countries) to 50fps, 576 liine video (used in most PAL countries) is fundamentally hard to do well, although there are machines now which do a pretty good job of interpolating the missing fields/lines. Certainly, the picture quality on American interlaced video programmes broadcast over here is noticeably better than it used to be.
NTSC, PAL and SECAM are methods of encoding colour on top of an analogue monochrome signal, so have nothing really to do with digital HDTV.