From the html Reference Guide (which I highly recommend):
set_report_severity_override
function void set_report_severity_override( uvm_severity cur_severity,
uvm_severity new_severity )
set_report_severity_id_override
function void set_report_severity_id_override( uvm_severity cur_severity,
string id,
uvm_severity new_severity )
These methods provide the ability to upgrade or downgrade a message in terms of severity given severity and id. An upgrade or downgrade for a specific id takes precedence over an upgrade or downgrade associated with a severity.