Getting new numbers is the simplest option and is recommended to avoid any hiccups and downtimes. However, if you need to keep your current numbers for any specific business reason (for example, customer support, brand equity), contact Sinch who can help you port them. It is possible to provision your landlines and long codes as 10DLC.
Note: Porting is complicated and can take a significant amount of time (depends on the carrier who owns the number). Sometimes the carrier may reject the port request, especially if it is to port SMS only. Full ports (Voice+SMS) are usually more straightforward.
Visit the Sinch 10DLC Product Page to learn more.