In wavelength (λ)-tunable wavelength-division-multiplexing/time-division-multiplexing passive optical networks (WDM/TDM-PONs), an optical line terminal (OLT) directs optical network units (ONUs) to change wavelengths to achieve functions such as load balancing or power saving. This demands λ-tuning without any data-frame loss, i.e., hitless λ-tuning. In addition, there is a need to increase the dynamic bandwidth allocation cycle period (DBA-p) to expand scalability as regards the splitting ratio or fiber reach. Unfortunately, with a large DBA-p, more buffer memory is needed in the OLTs or ONUs to cover the tuning delay demanded by the conventional hitless λ-tuning sequence, which requires a message exchange between the OLT and the ONU after λ-tuning. In this paper, we propose a novel hitless λ-tuning sequence to reduce the tuning delay. With the proposed sequence, the completion of λ-tuning is decided by calculation, and the buffers for upstream and downstream transmissions are released according to the calculation, instead of the message exchange. Experiments confirm that the proposed sequence can tune the wavelength of the ONU's transceiver without any data-frame losses for multiple measurements. Each measurement includes 300 λ-tuning operations. Additionally, it is shown that, when the DBA cycle is 4 ms, the tuning delay and buffer memory for upstream transmission can be reduced by 98% and 50%, respectively, while those for downstream transmission can be reduced by 86% and 86%, respectively, compared to the conventional sequence. We also show that the maximum upstream throughput is not degraded.