If your Sumtracker product is linked to only 1 listing, the SKU change will be done in the same product. The inventory will remain the same and all reporting, history will also be available linked to the same product.
But let's say a product in Sumtracker is linked to 2 or more listings (having the same SKU). And you change the SKU in one of those listings. Then a new product in Sumtracker is created in Sumtracker with the new SKU. We need to retain the old SKU as well since there still exists another listing with this old SKU.
You will need to update the stock of newly created SKU in Sumtracker and all its inventory history, reporting will start afresh.
In all cases, you will always find all SKUs you create online in Sumtracker.
How to change the SKU when it is linked to 2 or more listings on online stores while keeping all data intact?
Please go through this help article to see how to do this.