I’ve tried to set it using R-click Open With/Other, choosing SU 2017 from the application list, and clicking Always Open With.
That works once if I open a file, then close it, and double click the SKP file again. But if I close the application again, or click on a different file that is v2017, it tries (and fails) to open with SU 2016, which is still showing as the default app on a R click.
Any other options to get the selection to ‘stick’?