Welcome to the community!
If your old SIM fits in your new phone, you can go ahead and use your old SIM. The new SIM is provided with new devices in case a customer's old SIM does not fit in their new phone. In those cases, they would need to update their account with the new SIM number. Since you are able to use your old SIM, you are good to go! You can keep the new SIM in case you need to update your SIM in the future.
Hope this helps 😀