It found that the trend for large outsourcing deals is accelerating in Europe and that such upheaval is changing the type of staff and skills in IT departments.
Forrester divides an IT department into six sections: client management, project management, development and integration, infrastructure and service delivery, technology, and IT management and administration.
The rise of large outsourcing deals has meant that some IT roles disappear or decline, according to the report's authors Richard Peynot, Andrew Parker and Sonoko Takahashi.
Roles that transfer to suppliers include application development and integration and infrastructure and service delivery.
But some roles remain in the user's IT department and will become more important after the outsourcing agreement, according to Forrester. These include technology strategy and architecture, IT procurement and staff to manage the outsourcing supplier.
"Few firms already have these functions [supplier and contract manager]," said the report. "They become critical for managing and controlling providers that run a firm's IT infrastructure. These roles are particularly important in phases such as reversibility, contract renewal and extension and migration plans, for example, migrating desktops to Windows XP and servers from Windows NT to Windows Server 2003."
Forrester said companies that have outsourced or are about to should compile a "skills map" and set up education and career development plans for remaining staff.
It estimated that between 30% and 80% of employees may require an education plan. Certain disciplines such as regulation and contract management will require a formal academic curriculum, and in other areas - such as service level agreements and supplier management - internal education sessions by consultants will often be effective.