## 2 Related Work  
Based on the consideration of CAPEX and OPEX [15], a one-time deployment of flex-grid technology for the entire network may not be economically viable. Consequently, the brown-field deployment network, as the basis of the existing fixed-grid network in which both fixed-grid and flex-grid technologies co-exist with seamless interoperability, is a more realistic scenario [16; 17]. In fixed/flex-grid optical networks, many technologies have been applied to support communication between WDM nodes and elastic optical network (EON) nodes, such as the virtual concatenation technique [18]. At the same time, to address the problem of spectral path allocation, dynamic resource allocation algorithm with mixed grid recognition considering the interoperability limitations was designed and reported in [19]. The gradual evolution from fixed-grid networks to flex-grid networks, called migration, has been widely investigated [20; 21; 22; 23; 24; 25]. Authors in [20; 21] studied various migration options from fixed-grid to flex-grid and different impacts on flexibility. With regard to the planning of physical layer constraints from fixed-grid to flex-grid optical networks, details are available in [22]. In [23], the author proposed an energy-efficient traffic grooming scheme to solve the problem of high energy consumption in fixed/flex-grid optical networks. In our previous investigations [24; 25; 26; 27], the problem of how to select fixed-grid nodes existing in network upgrade to flex-nodes and related routing strategies have been studied.  
When the process of network migration begins (either by upgrading the fully reconfigurable optical add-drop multiplexer (ROADMs) [25], or by upgrading the WSSs [26]), the existing services in the network will be interrupted, causing a large amount of traffic-loss since the fiber capacity is extraordinarily huge. In such fixed/flex-grid optical networks, to establish stable connections for services, various traditional protection techniques have been extended in optical migration scenario [28]. By applying the method of establishing a pair of working and protection paths for the service, the problem of connection interruption can be greatly improved, such as choosing a node disjoint pair of paths. One of the paths is the primary path for data transmission, and the other is the backup path, similar to Dedicated Path Protection (DDP) scheme [29]. When migration begins, the service carried on the primary path is interrupted, but the service can be quickly restored by switching the primary path to the reserve backup path. Meanwhile, as a path-focused survival mechanism, Shared Backup Path Protection (SBPP) which protect the services through working and backup routes has been optimized in [30; 31] for better service protection performance and effective resource sharing. Nevertheless, the strategies directly derived from the traditional service protection strategies need to reserve spectrum resources for the working and protection path for data transmission which will introduce a certain amount of waste of network resources. Meanwhile, previous studies are unaware of node upgrade during the process of service provisioning. If we can take the issue of node upgrade into consideration during the process of service provisioning before the network migration happens, the traffic interruption and data loss can be effectively avoided.