After a number of years, you may decide that you want to adjust the scale used on your rubric criteria. The question, however, is whether this will affect all the past grades entered using that rubric criteria. Yes — changing the scale affects any grade that used the criteria, regardless of the year.
The posted proficiency level remains the same, but the scale values change and affect how a grade is translated by the rubric. The posted proficiency level is always the abbreviation from the scale; if report cards are looking at the abbreviation as a reference, they would likely change.
If you wish to keep prior scales intact, you should create an entire new rubric scale as well as rubric criteria and obsolete the old scales and criteria used. This process should be done in conjunction with Veracross, so you should generate a ticket to facilitate this transition.