FlyerTalk Evangelist and Ambassador: The British Airways Club
Join Date: Jun 2008
Location: Somewhere between 0 and 13,000 metres high
Programs: AF/KL Life Plat, BA GGL+GfL, ALL Diam, Hilton Diam, Marriott Gold, blablablah, etc
Posts: 33,193
You'll pay a change fee plus any difference in fare based on historical fares if your original booking class (likely I) is not available.
Note that some cheap fares can be "YUP" or "WUP" in which case there may be slightly greater complications for rebooking (those are fares that are really normally for economy or premium economy but automatically book into business class).
You will need F award availability if you want to travel in F (or pay an upgrade to paid F as part of the change), otherwise you'll be back in J.