I found some documentation on the Union's website that says this:
11. Is there a time frame for redoing the matrix?
A. No. When circumstances create a large number of blanks on the matrix
due to conversions, auxiliary route assignments, etc., management should
review the list to determine if some movement of qualified leave replacements
or cross training is needed. (Analysis of Changes National Agreement 1995-
1999 pp. 25)
12. When redoing the matrix, can management change all the route
assignments?
A. No. Should management decide that some movement amongst assignments
is necessary, only those qualified leave replacements desiring to change
assignments will be affected. (Analysis of Changes to the National Agreement
pg. 25)