The Shelf Name consists of two values: Stack-ID.Shelf-ID.
ONTAP assigns an arbitrary stack ID, starting with 1 for every stack found in the cluster.
Q: I assume you added shelf 20 to the same stack as shelf 10?
When ONTAP discovers new shelves, it should have just assigned the shelf to the existing stack ID.
In your case, did you have any problems with cabling or possibly temporary duplicate shelf ID's when you added this stack? That is the usual reason for ONTAP changing the stack ID of existing shelves.
In any case, it's not service affecting that the stack ID's change. Nonetheless, if you'd like to regain your stack ID 1 again, you can force ONTAP to rescan and re-enumerate the stacks and shelves by performing a takeover and giveback in both directions.