Consider the possibility that some end users (or even corp networks) may have hardcoded your hosts' translation into their hosts files or perhaps corporate proxy firewalls that allow access onto to whitelisted web sites.
They will continue to point to the old IP addresses until you shutdown the service and they call you to inquire why *you* broke your system :-) If this HTTP service has a GUI (aka: web page versus credit card transactions), you should put up a warning on the web page whenever it is being accessed via the old IP address.
