Originally Posted by uncleswede
Not sure if this is what you need but, if you change your project timebase to TIME (in Project settings) you can add Tempo Markers with different values wherever you like and drag them around without affecting the video playback (although the video's audio waveform may resize visually it won't change the playback)
Thank you, this was the answer.
I am able to insert a new time signature/tempo anywhere in the timeline.
As a follow up, if there is anyone else having this difficulty, the other thing I did was enabled the metronome, and in settings enabled "count in before recording" of 2 bars. Not preroll, as this counts in the previous tempo.