With a sit down machine you move the quilt under the machine and with a standup you move the machine over the quilt. Two different things. Can't see how one can do both.

OP is asking if there is a longarm (frame mounted) that you can change the height of so you can either sit (on a rolling stool or chair) or stand. You most definitely can sit to use a longarm, it's a matter of finding a frame technology that allows it to go low enough for the seat you are comfortable using.

Not necessarily. I've been looking into a Sweet Sixteen and they make a table that you can either sit or stand. In both positions with the SS you move the quilt under the machine.