Kix,
6061 is in the M23 group (ref para 4.2.2 of D1.2), and is further classified as a heat-treatable alloy by Note 5 of Table 3.1. Table 3.1 requires requalification of the WPS if you increase the preheat temp by more than 100 degrees F. Preheat is not an essential variable with regards to welder qualification, so you would not need to requalify the welders (ref Table 3.9).
Mankenberg