Upgrade to Runway: licence key

I think I have seen something about this but can't find the info now. Just upgraded an existing Perch v2 licence to Runway. Changed LICENCE_KEY in my config file to the new one. This is on my existing Perch site: I'm not ready to switch over the live site but will work with a copy dev one for now. I get an error.

include(/var/sites/ ... /public_html/perch/core/runway/inc/loader.php): failed to open stream: No such file or directory in /var/sites ... /public_html/perch/core/inc/loader.php on line 36 etc.

Change back to old licence key and get error: Sorry, your license key isn't valid for this domain.

What can I do?