I have a 78xx in a new slab… Because I submitted it for a reholder lol
So the thing is you will definitely get the new slab for any standard-sized Pokémon cards - regardless of Grading or Reholder - if you submitted to their office in Japan after July 2024. This was officially announced by their Japan office at the time so not a speculation.
This also means that you may get new slabs for any valid cert if someone out there reholdered the card in Japan. In addition, you should not assume all cards in new slabs are graded in Japan since that someone could also reholder cards which are originally graded in US.
I’m not sure when the US office will make the switch but all my submissions last year (with last one returned in November) are still in old slabs.
If you’ve been handling slabs a lot you just need to tap it once and you’ll immediately know it’s new since the sound from tapping one will be dull. For cracking one… I suggest tile nippers lol