In some cases when rubric is changed students need re-grading but this re-grading can be done automatically.
1) Add option for automatic re-grading if possible and implement it
2) Indicate on assignment grading form students who need manual re-grading

Levels of rubric changes:

0 - no changes
1 - only texts or criteria sortorders are changed, students probably do not require re-grading (but we still ask a teacher)
2 - added levels but minimum and maximum score on rubric remain the same, students still may not require re-grading
3 - removed criteria or added levels or changed number of points, students require re-grading but may be re-graded automatically
4 - removed levels - students require re-grading and not all students may be re-graded automatically
5 - added criteria - all students require manual re-grading

See also discussion http://moodle.org/mod/forum/discuss.php?d=193460