This article is from the Stardates in Star Trek FAQ, by Andrew Main email@example.com with numerous contributions by others.
Many people have noted that between ST:TOS and ST:TNG, as well as the stardate
system being revised, the warp factor scale changed. The warp scale had
previously had no upper bound on warp factors, but the new scale has a maximum
of warp 10, which is defined as infinite speed. (See the Warp Speeds FAQ for
futher information.) It has been suggested that both changes happened a the
same time, possibly as the result of the same new knowledge about the nature
of time, warp drive, and the interaction of the two.
However, it actually appears that the warp scale changed sometime during the
classic film series, well before TNG stardates were used. Consequently the
two changes can not have occurred at the same time.