Currently, you are limited in deploying newer versions of Revit by projects in previous Revit versions. To deploy 2018 you can no longer have 2014 projects unless your firm wants to invest the time in upgrading all of the older projects. This requires time, coordination with consultants and you run the risk of corruption during the upgrade. This will be worst with 2017 with the change in text size.
This seems an arbitrary restriction from Autodesk. Three years often does not cover the life span of a project and until upgrading is less risky it is an unnecessary mandate that will limit companies from embracing new versions.