The thing nobody tells you about switching back and forth between a P and J is that if you practice or perform seated *at all*, the neck location shift REALLY throws off your position jumps. As a lifelong J player who once bought an amazing sounding/looking P that I had to sell because I couldn’t play it, I was on your side from the get-go. This quote makes it make even more sense.
Anyway, unless you’re going for the Kurt Cobain gaff tape look, you’re in for a new Warmoth pickguard. If you ask nicely, I’ll bet they’ll put a neck J route in at the “correct” spot, and then you’re off to the races on the neck position route.
As a recovering 3-knob / dimed tone knob J player, I cannot recommend dual stacks (with 500K tone pots) enough. You’ll have less routing and less knob re-postioning this way, too. I also recommend the Fender J under-pickup shields. While you’re on the phone with Warmoth, get one of their wiring kits with the *shielded* bridge ground cable and run that. Shield the cavity, tie it all together and you will have no noise in any pickup setting.
Do you have any pickups in mind?