Just did what you are asking.
It of course updated the design on the directory, adminp, ect. However the 8.5 servers continued to run fine with it in place. Technically the instructions say to upgrade the managing server of the directory first. We did as you are looking to do. Upgraded our Traveler server to 9, the of course followed that up with a Traveler 9 upgrade.
We did bring over the updated templates for the directory, ect from the upgraded server to the admin server so design refresh would not put it back to a 8.5 template version.
Be warned, traveler icons turn blue when they update their devices. Some users have a hard time handling that.