IIRC they have CIWS, EW, and Super-RBOC, for defense and a 3 inch gun mount for surface action. Their single-arm missile launcher and weapons system was only capable of firing SM-I... I don't *think* it ever got the SM-2 upgrade??? Could probably punt the launcher and put a RAM cell in it's place, then use the below-deck magazine space for something else. They also have a hanger aft. Put an Apache on there and you've got a pirate-killer.
Built pretty solid too!
Durability
On July 14, 2016, USS Thach took over 12 hours to sink after being used in a live-fire, SINKEX during naval exercise RIMPAC 2016. During the exercise, the ship was directly or indirectly hit with the following ordnance: a Harpoon missile from a South Korean submarine, another Harpoon missile from the Australian frigate HMAS Ballarat, a Hellfire missile from an Australian SH-60S helicopter, another Harpoon missile and a Maverick missile from US maritime patrol aircraft, another Harpoon missile from the cruiser USS Princeton, additional Hellfire missiles from an American SH-60S Navy helicopter, a 2,000-pound Mark 84 bomb from a US Navy F/A-18 Hornet, a GBU-12 Paveway laser-guided 500-pound bomb from a US Air Force B-52 bomber, and a Mark 48 torpedo from an unnamed US Navy submarine.[10][11]
https://en.wikipedia.org/wiki/Oliver_Hazard_Perry-class_frigate#Durability
Most of that stuff would have been removed prior to laying them up. So much of the cost of bringing those ships back to life would be re-installing that weaponry/sensors on their mounts and testing. Not sure what state the engineering plants would be in.
they had a late SM-2 upgrade, but it still didn’t get to SM-1 capability. I’d recommend exactly what you’re saying and replace the 1-arm with VLS. Would get a greater capability out of it.
There’s plenty of billboard space to configure an AEGIS if they wanted, but I wouldn’t put that amount of money into it. Just upgrade the fire control for the VLS and leave it at that.
Other option would be to strip the flight deck and put a launcher there ... but again for the money it would be a waste. plush you would lose the ASW capability they were best suited for.