This full research paper presents findings from semi-structured interviews conducted with eleven K-12 teachers, mostly in STEM subjects, to investigate how a professional development (PD) program impacted their AI competencies, in terms of attitudes, perceptions and strategies of integrating Generative AI (GAI) into their classrooms. The results show that teachers improved their knowledge and awareness of different aspects of GAI, revealing five major themes: overall positive evaluations of the PD program, shifting teacher attitudes towards GAI, perceived pedagogical benefits of GAI including personalization and multimodality, implementation challenges, and commitment to fostering student AI competencies. The study makes three key contributions. First, it provides recommendations for future PD programs supporting teachers' AI competencies. Second, it captures teachers' perspectives within the Swedish context and enriches diversity of the empirical data in this field. Finally, the findings offer valuable insights for policymakers involved in strategic planning and resource allocations to support teachers' PD.