Yes, I have given them all a heads up and am now starting the major roll-out.
My clients are health care providers, and I couldn't put it better than this copypasta:
For example, when it comes to HIPAA and PCI compliance, the regulations state that entities must employ all security patches within a certain time frame (depending on the regulatory body). Thus, when software reaches its end of life, there are no more security patches and therefore at the end of life, it reaches it’s compliance limit. The software manufacturer is no longer looking after the security vulnerabilities of the software and will not issue any patches or bulletins to combat any issue.
Given the big change in HIPAA/HITECH that takes place in a few weeks I am standing firm on this one. XP has to go.