Changing the name of the grade changes the name of existing grades for the existing entries on all student records (past, current, and future years that had the translation table on the course record).   

The new grade displays anywhere student grades appear (Records, Grades entry, report cards, transcripts, and other grades reports).  

For example, if you rename A to Z, Z will appear everywhere A was previously entered. If the numeric equivalents, GPA equivalents, and Cut off values do not change, it will not affect grades calculations when calculations are run.
If you change the numeric equivalents, GPA equivalents and cut off values as well and then change the translation table on a course record it should automatically update any new grades input in FAWeb gradebook according to this new translation table.