Every original cartridge is sealed, but not permanently. Opening one, removing the old film, and respooling it with new or current Super 8 footage is possible today, no new hardware required. It’s real, it works, and it’s how the DIY path on the roadmap already exists as an option.
It’s also genuinely fussy work: the film has to feed smoothly through the gate afterward or the mechanism can jam, and getting the splice right takes practice. The full disassembly and refill process, step by step, is worth documenting properly rather than summarizing here.