I'm no health expert, so take what I say with a pinch of (low-sodium) salt. However, it does seem to me that the amount of calories you burn standing up for a few hours is not going to make much difference to your health.
There's something else to consider though: lipoprotein lipase. It's generated in large amounts by your leg muscles and is used to break down fat into energy. Sitting causes a steady decrease in lipoprotein lipase production in your legs. The longer you sit, the less lipase you produce.
At that point, losing weight becomes extremely difficult and you start craving sugar for energy. There's a theory of weight management that people who don't gain body fat manage it because, throughout any given day, your insulin levels go up and down, you store a little fat then you burn a little fat. The ability of your body to switch back and forth between burning sugar and fat is called metabolic flexibility. Sitting destroys that flexibility.
You can also "walk in place" to get the heart rate up. Good luck doing that in a chair.