In principle you could break the fare -- upfare SAN-MIA to the lowest permissible fare, and book MIA-MCO as a separate fare component though in the same ticket. There's no guarantee that that's cheaper than upfaring the whole itinerary.

Picking an arbitrary date and looking only at published fares (without checking inventory, which of course varies from flight to flight), I show the cheapest round-trip SAN-MCO as $222 r/t (in O), and cheapest qualifying bucket as $412 r/t (in V). In contrast, the cheapest SAN-MIA r/t qualifying fare (in G) is $580. This suggests you wouldn't get any savings from breaking the fare in MIA. Agent was probably right to encourage you to upfare the whole itinerary, though the agent might have misstated this (like in telling you "not possible" rather than "not cost-effective").