SOO was an US subsidiary that was owned by CP, much like GTW was a subsidiary of CN. In the mid-1990s both CP and CN extended their corporate image into the lower 48, and began painting SOO and GTW rolling stock and locos into their parent railroads’ colors and markings. IIRC, SOO and GTW still exist as “paper” railroads.
For the record, there are fewer than 150 SOO box cars left. A lot of them went to the Wisconsin Central when it was formed, but a lot more are just falling victim to old age. I doubt that CP is renumbering them into its own series.
It will continue to haul loads in this condition until it gets shopped for a serious mechanical issue - if and when that condition arises, CP wil then make the decision to either rebuild the car for more service or scrap it depending upon the needs of the railroad. If rebuilt it will likely lose it’s SOO identification and be given a CP number.