There are a number of reasons that the course should be changed after the race has started. If the wind shifts, the course should be adjusted to provide truer windward and leeward legs. If the wind becomes light and the race will take too long to complete, then it should be shortened either by a lap or a specified number of legs. If a turning mark has drifted or been dragged out of position so as to make the course different for a portion of the fleet, it should be replaced.
Moving Marks After the Start
It is perhaps more important in invitational regattas and championship events to alter the course when wind shifts occur. It is also easier to accomplish than it is in multi-fleet club races.
One RC boat should approach the new mark location carrying a spare mark. Another RC boat must station itself near the previous mark and hail the approaching boats of the change. They should indicate the new location of the mark. (For regattas this is done by displaying code flag C 
with either a red rectangular flag 
or a green triangular flag 
to indicate direction and producing repetitive sounds with a horn or whistle. As soon as the mark boat has dropped the new mark, they should pick up the old mark. There is a black band available to be placed around the new mark to identify it as the new mark. It is not necessary that the new mark be in position when the boats begin the leg as long as the RC boat is in position at the previous mark signaling the change.
A leeward gate may be adjusted at any time if the wind shifts less than 20° even if boats are already on the leg. You may shift one gate mark. Care should be taken that no boats are sufficiently close to be prejudiced. If the wind shift is greater than 20°, move both gate marks and use the notification procedure in the paragraph above.
Missing or Drifting Mark
In any kind of racing it is essential to deal with missing or drifting marks. Unless dealt with promptly and correctly, it can be cause to abandon the race as it may result in part of the fleet sailing a different course.
The quickest way to deal with it is to send two RC boats out quickly. One should anchor at the original mark location and display code flag M 
while making repetitive sounds with a horn or whistle. This advises the fleet to round him as if he were the mark. The second boat should retrieve the drifting mark and return it to it’s original location.
If only one RC boat is immediately available, send him up to move the mark.
If either the windward offset mark or one mark of the leeward gate drifts out of position, it should be corrected as soon as possible without interfering with approaching boats. Drag the downwind mark up to a position so that both marks are square to the wind.
Shortening Course
The Racing Rules of Sailing part 32.2 state the course may be shortened at any turning mark by displaying code flag S and making two sound signals before the first boat has passed. The finish line shall be between the anchored finish boat displaying code flag S 
and the blue finishing flag and the nearby turning mark. Or if at a gate, between the 2 marks of the gate. The RC should wait until the lead boat is within hearing distance before making the sound signal. The race committee can decide to shorten course at any time after the initial mark has been rounded. There is no requirement that the first boat be notified before rounding the previous mark. Although not required, hailing that “this is the finish for the “xx” fleet” may be helpful. The sound signal need not be repeated for every boat in a fleet. This should be repeated for each different fleet for which the course has been shortened. If possible, and only as a courtesy, station an RC boat near the mark prior to the new finish and hail competitors by voice that the course has been shortened.