Jitter is not only an issue of the playback but recording (digitizing) as well. The difference is that playback jitter can be suppressed while recording jitter cannot be removed and the only way to improve it is to digitize it again from analog tapes (if they still exist) with better more stable A/D converters (clocks). Some early master tapes were digitized poorly, but there are many other reasons for remastering. DSD, SACD, can only be mastered after conversion to 24/192, AFAIK.
Is remastered mainly just less jitter?
When a CD is remastered is it simply just less jitter???