Not sure what supplement you are covered by, but I'm assuming your language is probably similar to the Southwest Package Rider, which says,
"When a driver’s assigned delivery area is permanently changed by
fifty percent (50%) or more of its delivery stops, said driver shall
have the right to follow the major portion of the original delivery
area. When more than one (1) driver’s delivery area is affected,
those affected drivers shall be afforded the opportunity amongst
themselves to bid the areas affected in accordance with their seniority."
So, according to what you said earlier, you started with 100-110 stops. You lost 40 mall stops, and gained about (40 stops?) from another route. The 50% factors in not just what you lost, but also what you gained (from another route).
That being said, it sounds like the majority of your delivery area is outside mall work (60-70 stops), which would mean, even though your route has changed by over 50%, you have followed "the major portion of the original delivery area."
They kept you with the majority of your delivery stops, it's just not the part that you want, which is unfortunate.
I suggest talking to management calmly and explain why it is in their interest to have you at the mall as opposed to another person. Tell them it is safer to have you there, because you are accustomed to the environment, know how to manage walking through the mall with that many people, blah blah blah. If they say no, try talking to a steward. They can, from time to time, get management to do things, even though there is no contract violation.
Best of luck to you.