I am slowly getting back to cycling, after having had a neck injury (not bike related). When I injured myself, I had a brand new Myka sport which was left unused for two years.
I started out terrified and adamant at wanting a bike that let me put the feet on the ground from the saddle. Given that I am 5' tall, it was virtually impossible, even with the bikes designed for that purpose. Yesterday I tried a Specialized Expedition and I couldn't even reach with the tips of my toes. But even if I give up the idea of having a bike with this feature, I liked from the expedition two things: the super low step (which alleviates my fears of having to dismount to stop) and the more upright position (great for my neck) and the saddle.
I asked the salesman about the possibility of just changing stuff (mainly the stem or the handlebars) to my existing bike, rather than spending $500 in a new one. He made it sound like an impossible feat, not worth the effort. (He pointed at the shifters and breaks, and said "you'd have to change all that"). I am not sure whether he was realizing he was going to lose a sale or what.
So realistically, how hard and expensive is it to change a stem or a handlebar? Would a more upright position work with a mountain bike geometry? Does changing a stem change too much the handling of the bike? (I tend to suffer from "wobbliness" sometimes and some bikes are more wobbly than others). What kinds of stems should I consider?
fwiw, I recently bought and installed a Delta steerer extender. It made a very noticeable difference bringing immediate relief to my neck. The best $22 and 5 minutes I've invested in fitting my bike. It's worth a shot and if you buy from REI, you can always return it and they'll likely install it for you for free if you need them to.
Making any major change in a bike's steering system will change the handling of the bike. The Expedition is an unusual bike that is set up to ride in a very particular manner. I would not try to change the geometry of another bike to sort of match that of an Expedition.
One seldom gets off a bike to stop anyway. Most folks come off the saddle, leaving one foot on a pedal and putting the other one on the ground and leaning the bike toward the down foot. To start off, just push off with the down foot. Or you can come off the saddle, put one foot down and then the other. To start off, lift one pedal up with your toe, put your foot on it and push down.