@peekay123 is correct, a castellated version of the Electron would work (you’d need a cut-out in your PCB) and makes the most sense.
@rickkas7 is also correct in that a true “P1” version (with no components on the bottom) would require extensive rework of the Electron PCB.
Note, all three of us are just speculating and do not represent the official stance of Particle on this matter. Perhaps @mdma or @BDub an comment?