Not much, really. Assuming that the call center is stuffed adequately and was able to meet the previous service level of 80/80, the new target will only improve service level adherence. The new SL has no impact on any of the customer-facing metrics such as ASA and abandonment rate (assuming, of course, that the call volume and head count remain the same).
You can use a resource planning calculator (e.g.
www.DiagnosticStrategies.com/EasyErlang.html) to see the impact of changing service level target using your call centers call volume and staff.