How can I start a song in 3/4 meter for section A, change to 4/4 for section B and then change back to 3/4 for repeating section A? Thanks all.

This is an oddity in BIAB. To do this, a song must start with a style in 4/4. You can immediately use F5 in the first measure to make it three beats. Some people use a blank measure one (all instruments muted) and switch to three beats in measure two. You are unfortunately going to hear a count-in that doesn't maker sense.

What Matt says works, but that's really using a 4/4 style, but playing only the first three beats of the style. If you actually want to start with a "waltz"-oriented (3/4) style, then later switch to a 4/4 style, you'll need to select a 4/4 style to start (because the style sets the time signature), mute the first bar, then select 3/4 style at bar #2.

You can then switch back to a 4/4 style later in the song.

Just one of the things we love about BIAB.