In this case, the uncensored dialogue will have been taken from an exisiting, heavily compressed digital mono source (probably the very first Arrow dvd). The dialogue will have been upsampled (which will not in itself improve the quality, though further processing can) and integrated within the HD audio track.
The HD audio track will be a lossless (uncompressed) digital recording of the original audio (
AFAIK analogue in the case of Day) - likely rendered at 48khz (sample rate) 16 bit (bit rate), which is basically a bit above cd quality. Sample rates and bit-rates can be higher (the Akira blu ray is 192kz and 24bit). Most film audio is mastered at 24 bit 48khz these days, but with these old, pre-digital films, the analogue audio can be recorded and mastered at whatever rate is available (digital audio can too, but will only benefit if remixed with eq and audio processing tools running at a higher rate). So, even if a film is 40 years old, the HD audio can be recorded and mastered at extremely high rates (improving the accuracy of the digital recording, essentially).
The main improvement with a HD track is that the audio is not compressed. If you ever directly compare cd audio with a 128khz mp3, paying particular attention to the high end (eg. cymbals, acoustic strumming, etc), then you'll notice the higher the compression, the thinner and less detailed the sound. 5.1 mixes tend to sound much more immersive and convincing when uncompressed.