It *is* possible, just via CFW (for instance Pasta). The only options here are A) modifying the ROMs (not possible due to signature checks) and B) disabling the AP check/spoofing its results (requires kernel access). It won't work on "all firmwares" like the Sky3DS normally does, the hardware is just not capable of faking a check it wasn't designed to fake. Designing, manufacturing and releasing a new design is a costly endeavour, they wouldn't have done it if they could just upgrade the old cartridges, as some users said above.