If you get a re-clearance that is significantly different than what is on your master document, you will need to update the master document, get a new one sent to you, or you will have to compute a new one on your own. The math isn't that hard, but you are likely to be in a rush when this happens and that means you will be prone to making math errors. You should carry a supply of blank navigation worksheets on every oceanic flight. You can download a blank copy: Navigation Worksheet (Blank). Now that you have the worksheet, here are your options:
I've only had to do this a few times and each time it was with the plotter and circular slide rule. It was tedious and it was not as easy as I remember from my lieutenant days. The next time I will pick up the phone. Why bother? There are reports every month of a crew that got a re-clearance and ended up flying into the wrong airspace. It is nice having an accurate master document to check the electrons flying the jet.
What follows comes from the sources listed below as well as my comments and techniques shown in blue.
[AC 91-70A, ¶3-6.z.] Scrutiny groups determined that a re-clearance scenario is the greatest contributor to an oceanic error (e.g., GNE, Large Height Deviation (LHD)). Experience suggests that when ATC issues a clearance involving rerouting and new waypoints, the risk of error increases. The procedures used to copy the ATC clearance, load and check the waypoints, verify the flight plan information, and prepare a new plotting chart should be the same as the procedures for beginning a flight. Designate one pilot to fly the aircraft while the other pilot reprograms the navigation systems and amends the cockpit documents. In the event that a re-clearance involves a direct routing, retain the data relevant to the original route in case ATC requires the aircraft to return to its original course.
[AC 91-70A, Appendix 2, ¶2.f.(5)] A re-clearance (that is different from the oceanic route requested with the filed flight plan) is the number one scenario which leads to a GNE. Crews must be particularly cautious when receiving a re-clearance. Both pilots should receive and confirm the new routing and conduct independent cross-checks after updating the LRNS, Master CFP, and plotting chart. It is critical that crews check the magnetic course and distance between the new waypoints as noted in PREFLIGHT under the paragraph "LRNS Programming."
[AC 91-70A, Appendix 2, ¶2.c. LRNS Programming] (4) Track and Distance Check. To minimize oceanic errors, it is important to conduct a magnetic course and distance check from oceanic entry to oceanic exit. Operators should establish a tolerance such as + 2° and + 2 NM. The course and distance check comparing the Master CFP against the LRNS are critical in detecting errors you may not have noticed by simply checking coordinates. A difference of more than 2° between waypoints may be due to a difference of the magnetic variation in the database versus the variation used in the Master CFP. Recheck and verify any difference outside the + 2° or + 2 NM.
(Click the sheet to download a copy)
Portions of this page can be found in the book International Flight Operations, Part VIII, Chapter 28.
Advisory Circular 91-70A, Oceanic and International Operations, 8/12/10, U.S. Department of Transportation