When switching from one WordPress theme to another all your content will remain the media library, your pages and posts will also still be present, only theme specific items may not work with a new theme.
Switch by Using our
Switching from ProPhoto has never been easier. With our new PP FloSwitch plugin you can convert your old ProPhoto galleries and grids so they can be used with any theme.
FloSwitch is compatible with PP4/PP5.
Also you may want to check out Flo Launch plugin.