Doesn't hurt to removed all the gravel, drain 90% water out, removed the fish to a bin, add soil and wait till water clear out. Could take from 30min to a hr b4 water clear. Add back the gravel, fill tank back up, turn on filter, wait so more. Add fish back.
All the good stuff is in the gravel, as long you use the same gravel u won't have to cycle the tank again.