With the growing prevalence of HDR-capable cinema venues such as Cinity LED theaters, there is an increasing demand to convert existing Standard Dynamic Range (SDR) films into High Dynamic Range (HDR) formats for theatrical presentation. However, existing SDR-to-HDR conversion methods are primarily tailored for consumer-grade content such as television and therefore fall short of the stringent requirements of professional cinematic material. To bridge this gap, we present HDRMovie7K, the first large-scale, lossless dataset of cinematic SDR-HDR frame pairs sourced from professional Digital Cinema Distribution Master (DCDM) workflows. Based on this foundation, we introduce HDRMovieformer, a transformer-based framework featuring a Luminance Estimator module for luminance guidance, a Luminance-Guided Multi-Head Self-Attention to focus on critical fine-detail recovery, and a Chroma Refiner for color accuracy, optimized with a novel Wide Color Gamut Loss. To further evaluate our model in online streaming media scenarios, we introduce HDRMovie1K, a dataset curated from publicly available HDR film clips. Extensive experiments on both HDRMovie7K and HDRMovie1K demonstrate that our method achieves state-of-the-art performance. The source code and pre-trained models will be available.