Microsoft is making changes to its MSN instant messaging service that will lock out users of third-party software that uses the service as well as users of older versions of Microsoft's own Messenger client, the company said Tuesday.Microsoft\u00a0is making changes to its MSN instant messaging\u00a0service that will lock out users of third-party software that uses the service as well as users of older versions of Microsoft's own Messenger client, the company said Tuesday.Users have to upgrade to the latest versions of MSN or Windows Messenger by Oct. 15 or they will no longer be able to log on, Microsoft spokesman Sean Sundwall said. The upgrade is required because of "security issues" with the older versions of the Messenger clients, he said, declining to specify those issues.Besides those older Messenger clients, the move also affects IM software such as Trillian, Imici and Odigo that allow users to consolidate multiple IM accounts in one client, Sundwall said."It is our expectation that those who use our service with unlicensed or unauthorized third-party clients will likely not be able to log on after Oct. 15," Sundwall said. "We would encourage those third parties to contact us to work out agreements by which they can continue to have their customers access our network."Sundwall would not comment on what type of agreement Microsoft would want with third-party IM software providers. "We are very interested in interoperating with all third parties, there just needs to be a formal agreement," he said.However, Trillian users who posted comments to an online forum are hopeful. Though current versions of the software may be affected, a new version is said to support the latest IM protocols also found in Microsoft's MSN Messenger 6, according to postings to a forum on Trillian's\u00a0Web site.Microsoft on Monday sent out e-mail alerting MSN users to upgrade. Some MSN users reported receiving a dozen or more copies of the same message. The message resembles a hoax or prank. The first line is all capital letters: "ATTENTION: IMMEDIATE ACTION REQUIRED FOR MSN AND WINDOWS MESSENGER USERS."Also, users are told that their IM service will be cut unless they upgrade: "NOTICE: If you are not using an updated version, you will be unable to continue using your MSN Messenger or Windows Messenger Service."In fact, the message is genuine and sent by Microsoft to all users of older Messenger clients and third-party clients that to Microsoft's network look like older Messenger clients, Sundwall said. A relatively small number of people got multiple e-mail messages because of a "snafu" in Microsoft's e-mail system, he said."We sent out this e-mail because it is important that people understand that there is a new security standard in the newer Messenger clients," he said. "If you get the e-mail, that means that you have to upgrade."Microsoft regularly sends e-mail to its customers, according to Sundwall. However, the company never sends e-mail with attachments because of the security risk. Where possible, Microsoft electronically signs the e-mail, he said.Microsoft claims about 100 million unique users log on to the MSN Messenger service each month. Less than half of those use an old client, according to Sundwall.Users of Messenger clients older than MSN Messenger 5.0, Windows Messenger 4.7.2000, or MSN Messenger for Mac 3.5 will be locked out, Microsoft said. To check the version number in MSN Messenger or Windows Messenger, users should select the 'Help' menu in Messenger and then select 'About'.Microsoft will keep reminding users that they need to upgrade. Beginning Sept. 18, users who have not yet updated will receive notifications in Messenger that they need to update before Oct. 15, Sundwall said. More information on the upgrade is available\u00a0here.