I think the thing to do is to talk to someone who knows a lot about the type of machine you want to convert, perhaps a factory technician or district rep. I asked once whether my Innova could be converted to a sit down (if for some reason I moved to a place where the 12 foot frame wouldn't fit) and the answer was complicated. Yes, it could be converted, but it would actually be better to trade it in for a sit down machine because of the expense and the loss of features.
The tall chairs (usually saddle chairs) may be helpful to someone who is doing detailed work, such as stippling, because some time may be spent in the same spot. However most work at the longarm requires moving smoothly and frequently back and forth along the width of the frame. Stopping to use your hands to move the chair along makes this very difficult.