First, making sure that all tracks are in Musical Timebase, and that any audio clips are set to Musical Mode in the Pool (with the correct initial tempo), change your time signature at the desired measure, then, at that same position, insert a tempo event that is 2/3 of the first tempo (e.g. if your tempo when in 6/8 was 120 BPM, change it to 80 BPM when in 4/4).
Don’t forget to set the Score Display Quantize to show 8ths triplets during that 4/4 section 
