I've had no issue with splitting my itinerary and then changing just my flight, did it twice.

Best bet is to call the Mosaic line and explain to them that you had booked the trip before you knew there would be a work conflict. Rest of the party is still going on the original itinerary and you need to change your date, they'll even help you make that change on the same call.

I never split the itinerary and then cancelled my flight though, that's an outright abuse of a Mosaic benefit.