The TNG Technical Manual explains the change, it went from a cubic function to a logarithmic function along with some technobabble justification for why. There’s also some interesting background info and formulas on Memory Alpha, apparently the scale change happened in 2312 according to some magazine.
They certainly exist separate of each other. That we all have to call into question the timelines is half the fun.
Concerning times they exceeded warp 10 in TOS:
The narrative awareness and scaling has changed over time. I imagine largely due to people like us.
The TNG Technical Manual explains the change, it went from a cubic function to a logarithmic function along with some technobabble justification for why. There’s also some interesting background info and formulas on Memory Alpha, apparently the scale change happened in 2312 according to some magazine.