S9 and S9js are the same hashboard size, S9i is different, it's a bit smaller (the hashboads heat sinks), my information is accurate dating to as far back as maybe 2-3 years ago when I last laid my hands on anything S9, so take it with a grain of salt.

However, if you manage to physically fit them, they will work without a problem even with the stock firmware, I still remember scammers putting S9 hahsboards in S9j because the J was selling at a bit higher price, the S9i was always the most expensive among them all because it was unique and they couldn't fit S9 hashboards in it.