I think what's already been suggested, and what you've already talked about doing, is a good idea: take the upright bike out for a nice, long ride. Maybe do an overnight trip, to see how it works loaded up.
I always thought I'd like to be as upright as possible, but I ended up finding a happy medium between bolt upright and hunched over drop bars.
The problem is the longer I spend with all my weight on the saddle, the less comfortable it is. For shorter rides, I like an upright position. But sometimes even on a short ride there's wind and maybe rain that you don't want hitting you full in the face.
I don't have any photos of me on the bike, but here's my little Tote/Cycle, may it rest in peace.
It was very upright, and lots of fun to ride. Super stable, too. I don't think it was all about seat to handlebar height, either. The distance between from the seat to the bars is also a factor, and for bike stability, I think the seat in relation to the wheels is a factor. The Tote/Cycle seemed to put my weight close to the center of the bike, and I think that helped make the ride more solid without me making an effort to support some of my weight up front.
I loved riding it around town. My five mile commute was fine. Shortly before I found that bike, my wife had bought me a new hybrid and she used to get annoyed that my hybrid sat at home while the 30 year old Tote/Cycle got all the ride time. But I probably never road it more than 30 miles in a day. It got less comfortable the longer I rode it after 10 miles or so. It was great for bopping around down town where I was on and off the bike all the time, but it wasn't great for a long stretch of riding.
Then there's my Long Haul Trucker.
Bars are lower. definitely more stretched out. Not as low or as stretched out as drop bars, but it means I can move some of my weight forward. Much better for riding all day. LHT is still a really stable bike, but you can feel it when there's a lot of weight in the back and none in the front. It's not a good feeling. But with the right bar positioning, you can shift a little weight forward and all is well. I was pretty sure I wanted to be as upright as possible when I got the bike. I started out with the bars as high as possible, the flipped them to get them a little lower while improving the angle of the bars, and then began slowly moving them down. There were a couple of spacers above the stem when I finally disassembled that bike the other day. I never felt like I couldn't see the world around me, but I was definitely not upright, but nor was I laid down across the bike like I was in a race. It was surprising because I went in assuming that a bolt-upright position was exactly what I wanted.
Still, if I was not going to spend hours at a time without stopping, I could see a more upright bike being fun. So give it a try. Put in some real miles and see how it feels. And in the end, the best bike to ride on a tour is bike you enjoy being on. Good luck.