When classic rock bands, like Led Zeppelin, Black Sabbath, Deep Purple, have converted their vinyls to CDs, was that the time of "remastering" or is it possible to buy an un-remastered CD in stores? I bought Led Zeppelin 1 and I'm wondering if it's the remastered version.
It will say "remastered" somewhere about the box or sleeve.

if it doesn't, the original sound has still been adjusted for CD, because obviously disc,vinyl and cassette all have different tones, which have to compensated for.

The earliest CDs weren't remastered at all, and a lot of them have a hum on the high end.