To cop with the exponential growth of network traffic nowadays, migration from fixed-grid to flex-grid technology in the backbone optical networks (ONs) gains broader attention. On the other hand, virtual optical network embedding (VONE) emerges as the potential solution to communicate between remote organizations and share physical network resources simultaneously. However, the methodology to analyze and optimize VON over mixed-gird networks is still an open problem. Consequently, the analysis of different migration strategies from fixed-grid to flexible-grid networks is also unknown. In order to cope with the aforementioned problem, this paper proposes an online VON resource allocation model over mixed-grid ONs. The proposed model uses a simulated annealing (SA)-based heuristic algorithm to embed VON connections over mixed-grid ONs by performing node and link mapping. The performance of the proposed heuristic is evaluated over NSFnet network in terms of blocking ratio and total power consumption for different migration strategies and spectrum allocation policies. Simulation results indicate that blocking ratio and power consumption is minimum when increasing flexible island-based migration strategy is favoured over other two considered migration strategies.