Optical Packet Switched Metropolitan Area Networks (OPS MAN) are among the most promising solutions for Next Generation MAN architectures. As far as the network synchronization and the packet format are concerned, compared to an asynchronous MAN that supports packets of variable size, a synchronous network with large fixed-size packets offers a significant gain in the network throughput. It avoids bandwidth fragmentation and reduces the number of generated optical headers [1]. In such systems, client packets of variable size are aggregated and accommodated into optical fixed-size containers (fixed-size packets). In this paper, we show how delay constraints and the lack of segmentation mechanism may lead to the creation of optical fixed-size containers which are only partially filled with client packets. When optical containers pass intermediate without O/E/O conversion, the remaining unfilled space in such containers constitutes a wasted amount of bandwidth. Therefore, we propose a novel mechanism that improves the filling-ratio of optical containers. Our algorithm (so called DCUM for Dynamic CoS-Upgrade Mechanism) is based on the use of timers, which values change dynamically, in order to create containers with high filling ratio while limiting the time needed for their creation. We investigate the performance of our algorithm through simulation works. Our experiments are performed on an Optical MAN network with a ring topology. Numerical results show that, compared to existing solutions, DCUM provides optical containers with high filling ratios, and thus keeps the network performance (in terms of packet loss ratio and mean access delay) at safe-levels, regardless to the network load and the timeslot duration (transmission time of one optical container)