You do not have a choice. When those users migrate to Win 7 / Win 8 they must upgrade. As for you testing IP Agent on a Win 8 machine, do not bother becasue it does not work. If you must test / troubleshoot user issues then I would suggest you get a second machine with XP on it.
Sorry to be a downer on this but we have already been there, the old application will not function with the newer operating systems and Avaya has no plan to make it so. I wish they would because I find IP Softphone / Agent to be much better than the one-X series of applications.