Your VFR flight plan amendment is a SAR issue.
If you go missing, they will listen to the ATC tapes.
For that reason you should advise flight service " ABC VFR flight plan amendment". They will tell you to go ahead then just advise divert position Time/altitude/time for each subsequent position and that's it. It will simply be recorded.
Your Instructor should really have done that with you. You won't need to do a nav to cover it though I wouldn't have thought but don't you have to do a diversionary flight test?
Guard
