In the preamble to the Race Signals section of the RRS (inside back cover) it says:
When a visual signal is displayed over a class flag, the signal applies only to that class.
Under "Changing the Next Leg" in Race signals, it says:
C: The position of the next mark has been changed
So
theoretically, there doesn't need to be anything in the sailing instructions - even at the start.
However, since it is a rarely used application (change at the start) it was wise of John Macielag to call the competitors over and explain it to them before he did it. That's probably better than putting it in the SIs (If you want to keep a secret from the sailors . . . )
The whole point of racing is to have
fair racing, not to confuse the competitors. The starting signals should not be a test of their rule book knowledge.
