I installed Syndey Pro on a Sydney version that I had changed the header slider images on, those images reverted to the demo images !!
The customizer’s settings have done with the free version can’t instantly be transferred to the pro’s customizer. Thus you have to do these steps: https://docs.athemes.com/article/111-upgrading-from-sydney-to-sydney-pro
For some cases, you don’t need to rename the folder name as mention in the third step. Just do export-import.