Bicycle Tutor

I have a Trek 1.2 road bike with a threadless headset and I want to replace the stock stem with an adjustable one. The idea is to ride low and aggressive at times and then change over for a more comfortable, upright and casual ride with my grand kids. What I'm wondering is does the stem slip over the headset tube or do I need to loosen the headset to replace the stem?
Thanks, Dan...

You don't need to do anything to the headset to remove the stem. To take off the stem, you will remove the top cap completely and loosen the 2(?) bolts on the back of the stem that clamp it on to the steerer tube (what you're calling the headset tube). The stem will then slide off the top of the fork.
NOTE however that once you remove the stem, the headset and fork are not really secured to the bike. If you life the bike, the fork can slide out and drop and the headset will come apart. Just be careful until you install the new stem.
To install the new stem, slide it onto the top of the fork (thin coat of grease on everything). The top cap is there to adjust the headset. So you tighten the top bolt until there is no play in the headset. Then the bolts on the back of the stem lock everything in place. Don't try to adjust the top cap if the locking bolts are tight or you'll just strip it. The stem has to be able to slide up and down on the steerer tube when you're adjusting.
good luck

Thanks Dave, for a couple of things. One, the correct procedure to install a new stem and the caution about messing with the headset. Two, the correct nomenclature for the steer tube.
Thanks again Dave.
Dan...