Here's the story:
The original layout of the Plunkett class cruisers mounted 2 anti-fighter missile turrets in place of the 2 particle cannon turrets on the center pylon. (...)
Shortly after her commisioning, the Plunkett and two sister-ships (Cernan, Borman) were involved in a brief skirmish with a group of pirate corvettes. Through the course of the engagement, the Cernan and the Borman were destroyed as the faster and more manueverable corvettes repeatedly flanked the two cruisers.
The Plunkett was eventually able to force the remaining pirate vessels to disengage, and returned to port with heavy damage. Following this encounter, all the remaining ships in the class were scheduled for refit, which involved removing the two missile turrets, strengthening the pylon and mounting the particle cannons.
I found it a bit strange that two small AA missile turrets were replaced by two massive Anti-Capship gun turrets with no reduction in any other field. Well maybe there were, I just don't know.