If displacements can go anywhere their seniority can hold, then there are only about 15 guys on MSP200 CA who can't hold MSP900 CA. Now, of the remaining people who can hold MSP9CA, about 20 people who are lineholders on the 200 who get knocked back onto reserve. Probably.

Worth noting, EV has been doing more and more MSP dual class flying as of late so it's possible we could see enough MSP flying to house almost all of the displaced 200 drivers on the 900 without changing bases.