Generally, it is possible to change the stock silencer on a Royal Enfield Bullet at the time of delivery, but it ultimately depends on the dealership's policies and any local regulations or restrictions.
Many motorcycle dealerships offer various aftermarket exhaust options or customization services that allow customers to change the stock silencer to a different model or brand. Some dealerships may offer these options as part of their accessories catalog or as add-ons at the time of purchase.
If you're considering changing the stock silencer on your Royal Enfield Bullet, I recommend reaching out to the dealership from which you plan to purchase the motorcycle. They will be able to provide you with specific information regarding their policies, available options, and any legal considerations you should be aware of.