Yes, you can definitely replace your old SFP-5UD with the newer MS-5UD-3 model. The MS-5UD-3 offers several improvements over the SFP-5UD. Firstly, it provides enhanced functionality with its five-zone capability, allowing for more comprehensive coverage. Additionally, it operates on 24 volts and 120 volts AC power, providing greater flexibility in installation. Moreover, the MS-5UD-3 is equipped with updated features and technology, ensuring better performance and reliability. Overall, upgrading to the MS-5UD-3 is a wise choice for modernizing your fire alarm system.