You do not need to do anything to the fork. There will be a problem with bottom bracket clearance and pedal strikes by going to the smaller wheels. Bikes that are typically marketed to run both wheel sizes, are designed with the idea of running standard 29" tires and has the frame clearance to also run a wheelset with 27.5+ tires. By 27.5+, we are talking 2.8 or larger size tires, with the larger tires making up for the smaller wheel diameter and thus ending up with a similar bottom bracket height as something like a 29 x 2.35 tire. The Fsi is an XC race bike, not intended to run both sizes and I seriously doubt you are going to fit anything big enough to offset the difference in wheel size.