Picked up a G35 a few weeks ago for USPSA and was checking the 2020 rules.  It appears as thought the rules allow for OEM or aftermarket barrels, but the part about the same length and caliber are gone.  Im fairly new to the sport, and wont be a threat to anyone, but wanted to confirm it now seems as though I can shoot a 9mm conversion barrel in my G35 if i desired.

Thanks

As I understand the rule change, yes you can put a 9mm conversion barrel in a G35 and shoot Production, assuming all other requirements are met.    
I understand extended/threaded barrels are ok, but will still have to fit in the box.   Depending on the length of the barrel, probably ok on a G19, maybe ok on a G17, not going to fit on a G34/35

This is actually a good way to get more weight in the right places on a Glock.   The G35 slide is slightly lighter than a G34 due to the barrel opening.   And a conversion G35 - 9mm barrel is heavier than a G34 barrel.  
With the new 59oz weight it will be interesting to see where people can add weight to a Glock.   Tungsten all the things...

It will still have to fit in the "box", unless it is Open or Carry Optic.