Hi, I wouldn't call it a "change", but rather a layering-in of something new and different. No one says you have to abandon your Bikram practice to practice Kundalini. But yes, it is going to be very different, and it won't just be the lack of heat.

I consider Kundalini (as taught by Yogi Bhajan, the way my teacher teaches it) a "shortcut" method to the energetic changes and effects that happen during the course of a yoga practice. It's more breathing and mudra (hand positions) than asana, much more, and the kriyas (exercises sets) are about a specific intention or purpose. The exercises are timed specifically, such as two minutes of this, one minute of that, and there is no adjusting. Most of the class is down with eyes closed, except of course when you need to see exactly what the mudra or movement is that the teacher is explaining and you don't quite understand, you're "allowed" to peek. Sometimes the kriyas as very tough physically (such as holding plank for three or so minutes while imagining the person you're most angry at in life -- one of my favorites! ), and sometimes they're just turning the head from side to side, inhale one side, exhale left. Oh, and the movements are fast, as fast as you can perform them without losing the form. Which means the breathing gets fast, which is how the energetic and other subtle bodies become affected.

I don't know if any of this makes sense, or if other people will completely disagree with me, so I guess the best thing is to go to class with no expectations. Actually, that's the best thing to do regardless!

Hope you enjoy!