While it is possible to upgrade to a new version of the KeyServer without forcing your users to upgrade to a new version of KeyAccess, each new KeyAccess version introduces new features and contains all KeyAccess bug fixes to date. You may want your users to have recent versions of KeyAccess so that they can benefit from these features and fixes. KeyAccess versions are controlled in the dialog pictured below.
By typing a value into the text boxes you can configure which versions of KeyAccess are up-to-date, out of date, or obsolete. Users who have obsolete versions of KeyAccess installed are told that they must upgrade in order to use managed programs. They will not be able to use managed programs until they get a new version.
Users of older but still valid versions of KeyAccess are told that they should upgrade to the newer version. Most managed programs are still usable, but if a program requires the services of a newer version of KeyAccess, users are told that the program can't be run.
There are various client versions in which new functionality was introduced, which may make these versions a natural choice for a cutoff.
The version control setting will cause a dialog to appear on client computers, but whether your users contact you or not is up to them. You may wish to sort the Computers window by Version, in order to proactively identify computers with old versions of KeyAccess.
Note that if you are able to deploy software, updating KeyAccess is very easy. The KeyAccess installer can be customized in such a way that a silent install can be done remotely, e.g. via GPO - for more details read the Deployment documentation.