In the music I received it’s written in 9/8, but really should more logically be notated perhaps in 3/4, with each of the 8ths from the 9/8 version becoming an eighth note triplet in the 3/4 version:
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Is there some “magical” way to convert the entire score this way in one fell swoop??
Yes, the time-signature can be changed in one go but this won’t convert eighth notes to triplets.
Select the time-signature in your score by clicking directly on the time-signature.
Go to the right-hand side panel and click on the 3/4 icon to open the time-signature panel.
In the panel select the time-signature you want, in your example case click 3/4.
The time signature in your score should now change and your score should adjust appropriately just keeping the eighth notes as they were.