Sorry but I would be reluctant to swap out the P3SE antenna for the P3S antenna. You'd be stuck in 5.8 ghz, and also you'd likely be limited in distance. The P3 SE, and A/P antennas are a significant upgrade from the old version P3S and P2V 5.8 ghz antennas. IMO, best option is to simply replace the cable, if possible, while keeping the existing antenna. IIRC, there's a circuit board in the leg where you'd expect the antenna to be, and the antenna cable might connect to it if it has the right end (sorry, I don't recall how it's connected). Or better yet, swap out for a good P3SE antenna and cable if you can find one. Usually, when sold used, they are sold with the legs - as one part. Legs, antenna, cables, all one part.
Like this:
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Or brand new antennas with cables attached (excluding legs) like this:
Compatible with DJI Phantom 3 Professional and Advanced RC FPV Quadcopter. Suit for: DJI Phantom 3 Professional/Advanced. With high quality and durable performance. R/C Helicopters. R/C Airplanes. R/C Car and Trucks.
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But I think it would be very difficult to source a good P3SE antenna (new or used, with or without legs) because P3 SEs are very rare relative to the other Phantoms.
Also, know that the P3 A/P are 2.4 ghz only. So using the P3 A/P antennas might work provided you were willing to stay in 2.4 ghz.