Network Function Virtualization (NFV) is expected to be a key enabler as regards both system cost reduction and rapid service deployment. In the context of NFV, network functions are assumed to be implemented as software modules on cloud server clusters, which results in a disaggregation from dedicated hardware. However, these assumptions differ in access networks where Network Equipment (NE) is typically deployed without redundancy in terms of economic efficiency. Additionally, some time-critical functions should be implemented on NE to satisfy the stringent Quality of Service (QoS) requirements. Therefore, in realizing rapid service deployment, software updating has a fundamental issue in that it causes Optical Line Terminals (OLT), which are NE in access networks, to reboot and this interrupts network services. Updating in-service OLT software without any service interruption is a key challenge for NFV in access networks. Recently, Dynamic Software Updating (DSU) technologies, have been developed, which update running software without interrupting the process in which the software is engaged. In this paper, we propose a way of applying DSU to OLTs and demonstrate in-service software updating without service interruption by using a prototype OLT. We also evaluate the performance of the proposed DSU technique for time-critical OLT applications. The proposed method shows the potential to update running OLT applications without interrupting services.