When I started BJJ, you would never let someone get a lasso guard on your arm because you were going to get bicep sliced. Now that's not a concern. To me that is a huge change, very similar to the no-reaping rule.
Guard jumping became guard pulling became butt scooting. The darce and the loop choke became super popular. Frames became the most important concept in defense. Getting underneath people became a huge concept in offense.
The paper cutter and baseball chokes basically disappeared. When I started the most popular choke from mount, knee on belly or standing was to take uncrossed grips on their collar and then spin yourself in order to cross your arms and get the choke. Anyone know what this choke is called?
You're no longer allowed to mash their face for control or submisions. No more rape choke, which used to be my favourite no-gi guard pass. No one does Amassa Pao any more.
The half guad game used to just be lifting them over your head with your arms/legs, but now it's an entire system. The knee slice became the most popular way to go through someone's guard, though I still prefer the same side knee slide stack passes. Actually, stack passes in general were basically replaced by running around the guard or x-passes.
Standing guard breaks took over, now you're told never to break guard on your knees. The wisdom back then was to never stand up in someone's closed guard, something Mario Sperry even says in his Vale Tudo series.
Giving up your back used to be the worst thing in the world. Thanks to Sakuraba and Eduardo Telles, it's now no big deal. (Also removing strikes to the back of the head probably had a lot to do with that. I think removing strikes is responsible for most of the changes in BJJ tbh.)
Wrestling off your butt in seated guards became the thing to do. The lying-on-your-back open guard is pretty much dead, but I still use it and regularly hit balloon sweeps on really good black belts who are not amused.
Z-guard / knee shield became a thing. I still remember the first time I was taught it, around 2004, and we had no idea what to do with it other than you could clamp your knees on their hip and keep them away. It was like an annoying half guard.