OK, I found a workaround.

1:
Set the song Style to _RYEGRAS

2:
On Bar 1, use <F5> (bar settings) to change the style to _CAPSIZE and the beats/bar to 3 (otherwise you get a 4/4 count in)
You probably get a message just to use the 3/4 style in the first place, but it's clear that this doesn't work, so just ignore it.
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3:
On Bar 9, use <F5> (bar settings) to change the style to __RYEGRAS and the beats/bar to 4
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Printing the Bar Settings should look like this:

**** Band-in-a-Box Report of Changes in the song at any bar ******
Song Title is: _CAPSIZE Demo - Capsize 12-String Campfire Waltz, File Name is E:\bb\AAA_MySongs\Development\_CAPSIZE DoesNotChangeTo4BeatsAtBar9.SGU [Sun, 31 Mar 2024 22:49:18]
Bar 1: TimeSig to 3/4.
Bar 1: All Choruses,,Style Change to: _CAPSIZE.STY

Bar 9: TimeSig to 4/4.
Bar 9: All Choruses,,Style Change to: _RYEGRAS.STY

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