You could try (did not test) :
- select the project (at bthe very top of file list in XCode left pane)
- in the inspector panel, select the first icon (identity and type)
- see which XCode version is selected and select 10
Otherwise, hope you have a saved version of the older project.
Did you try openningt the storyboard as a text or html file and see what could be wrong there (may be you can post the html version hre for inspection if not too large).