From I can tell, as soon as you have your newer version licence key the older key will expire (either 30 days or 60 days, unsure about which) thereafter. Meaning thus: any previous already authenticated installations (2013 / 2014 / 2015 / 2016) will remain to function like normal but cannot be reinstalled and authenticated after the transition period, only 2017 (or for future reference, the latest version you have a valid key for) will be able to be reinstalled and authenticated after the transition period.
I hope this makes sense? Else head over to SketchUp website itself and find and read the licence terms and conditions for greater clarity.
My fault, I didn’t realize the grace period had changed since last year. Looks like there is no more time after you activate your 2017 license like there was for 2016.
Pasted from 2016 Pro Upgrade email.
Important! 60 days after you authorize SketchUp Pro 2016, your 2015 license will deactivate.
Once your old license deactivates, you can keep using SketchUp Pro 2015 on any machines you’ve already authorized, but you won’t be able to use the 2015 license to authorize any new machines.
Pasted From 2017 Upgrade Email.
Single User Licenses: You can keep using SketchUp Pro 2016 on any machines you’ve already authorized, but you won’t be able to use the 2016 license to authorize any new machines