The Coast Guard Cutter Legare (WMEC 912) weighs anchor near the Statue of Liberty in New York City, New York, March 17, 2024. (U.S. Coast Guard photo courtesy of Legare)
The Acquisitions Directorate (CG-9) website includes a photo of USCGC Legare (WMEC-912) and the caption “270- foot medium endurance cutter Legare begins service life extension program at Coast Guard Yard.” Normally clicking on the caption would lead to a post expanding on the title, but in this case, it takes you to the In-Service Vessel Sustainment Program page. That page gives us this information about the 270 SLEP.
“Service life extension program for the 270-foot medium endurance cutters involves targeted system replacement to address system reliability, supportability, obsolescence and interoperability. This work will include upgrades or replacements to the electrical power generation and distribution system as well as the main propulsion engines. The mission is to facilitate continued operations during transition to the offshore patrol cutter by extending the service life of 270-foot cutters for up to 10 years. Work began on one prototype vessel at the Coast Guard Yard in July 2021 and on the second cutter in April 2022. Full production is scheduled to begin in 2023.”
The intention is not to SELP all 13 WMEC270s, rather they did two limited prototype SLEPS (Seneca, WMEC-906, and Harriet Lane, WMEC-903), and plan to do six full production SLEPs. These 270s should be the last WMECs in commission as they are replaced by Argus class Offshore Patrol Cutters. Harriet Lane’s most visible change was removal of her 76mm Mk75 gun and Mk92 fire control system. A 25mm Mk38 remote weapon station was mounted on the bow on an elevated position. This change is to be applied to all subsequent 270 SLEPs.
Spencer, (WMEC-905) was the first full production 270 SLEP. Spencer was the first 270 to receive new engines. Spencer’s old engines were to be remanufactured and reinstalled on a subsequent WMEC270 SLEP, presumably the newly arrived Legare.
This is a long process. Work on Harriet Lane lasted 15 months, so I expect we will have at least one 270 in the Yard being SLEPed for the next five years.
Chuck Hill's CG Blog 
why is the 76mm being removed with its FC system?
I don’t know why the 76mm and Mk92 were removed, but I will speculate a couple of reasons.
There may have been a desire to reduce topweight.
It may have been an effort to reduce crew size. Probably need about 5 fewer crew members.
What will take the place of the FC system?
The 30mm has an electro-optical fire control system. I don’t know that anything has replace the air search capability that was part of the fire control system.