I remember Hasegawa releasing a 1/48 F-4S more than 12 years ago. It is hard to find. IIRC, it was one of the “Hi-Grade” kits, with white metal landing gear, vinyl tires & a PE fret. I’ve seen this kit for sale at a few shows, & it was always going for around $100. If you can get your hands on Hasegawa #09353 VX-4 “Vandy 1”, buy it. The box says it’s an F-4J, but the kit contains their F-4S fuselage halves (with molded on formation lights), the current mold J wings (recessed panel lines), AND the current slatted E wings (yup, two sets of wings in one kit).
Berny gave you good info, but in my opinion, the difference between the E & S slats in 48th may not be worth the time to correct them, especially if you leave them in the position molded on the kit (retracted). Lengthening the wing fence, or making a whole new one, is not that difficult in 1/48th. The formation lights can be easily made from card stock, or just use decals
You stated you have the old mold J kit, so I’m going to bet that’s the one with the raised panel lines & Air Force style rear cockpit. You’ll be left with an odd bag of recessed wing panel lines, & raised fuselage panel lines.
You & I share the same plans of swapping wings to make an early, non-slatted E, & an S, but Hasegawa saved me the trouble with the release of their Thunderbirds F-4E kit, with non-slatted wings, which in turn allowed me to have the parts to build another F-4S with the stuff I already have.
What decals are you going to use on the S?
Chris Ishmael, with 17 Hase F-4’s in the stash