Microsoft Tuesday confirmed that it will eliminate a licensing restriction that prevented customers from moving virtualized applications to a different server more than once every 90 days.

Microsoft also said it will begin providing new levels of technical support for applications running on virtual servers, but not those running on VMware’s hypervisor.

Update: VMware joins Microsoft's Server Virtualization Validation Program after all.

Server virtualization technologies allow virtual machines to be swapped from one physical server to another without disrupting end users. But under current Microsoft policies, customers must reassign a software license in order to make such a move, and can only do so once every 90 days.

That 90-day restriction will be removed on Sept. 1 for the most commonly used Microsoft server applications, including SQL Server 2008 Enterprise edition, Exchange Server 2007 Service Pack 1 Standard and Enterprise editions, Dynamics CRM 4.0 Enterprise and Professional editions, Office SharePoint Server 2007, and Microsoft System Center products. In all, 41 server applications are affected.

This move was anticipated by industry observers, as reported by Network World last week. Analysts believed the time restriction unnecessarily limited the system flexibility that makes server virtualization so useful.

Not all server applications are gaining the new, more favorable licensing structure, however. Forrester analyst Christopher Voce said one noticeable absence is SQL Server Standard edition. Microsoft is apparently trying to give customers added incentive to upgrade to the more pricy SQL Server Enterprise edition, Voce says.

“This was an evolutionary change to the Microsoft licensing that everyone expected,” Voce says. “They introduced the kind of flexibility that you would expect for Microsoft applications in virtual environments. But it’s not all roses for certain customers.”

Microsoft predicted that many customers will be able to reduce the number of licenses they have to purchase, since they won’t need licenses for every machine a workload might possibly be moved to. “You don’t have to have every target machine licensed ahead of time,” says Zane Adam, senior director of integrated virtualization at Microsoft.