What role did the thorough initial success of the rovers upgrade them to, moving beyond proof-of-concept explorers?

Planetary archaeologists.

The initial job, confirming the presence of water within the strict 90-sol primary mission window, was achieved so thoroughly and rapidly by both Spirit and Opportunity that it fundamentally altered their ongoing operational descriptions. Because the initial hypothesis was soundly validated, mission controllers pivoted the rovers from short-term objective confirmation to long-term exploration. This transition allowed them to transition from merely proving a concept to undertaking much broader scientific endeavors, such as mapping entire regions and analyzing complex geological features like the Columbia Hills. This significant shift in scope upgraded their function from proof-of-concept explorers to planetary archaeologists, capable of extensive, long-duration geological mapping.