Welcome tot he party!
Character lock status is controlled by the memory address 7F01DF. Each bit controls a specific character. 80 - Crono, 40 - Marle, 20 - Lucca, 10 - Robo, 08 - Frog, 04 - Ayla, 02 - Magus.
The Exchange Menu is controlled by the storyline counter, visible from Window -> Misc Settings -> Manage Party (Location/Overworld).