Do you suspect entire room ceilings were lowered for ductwork? That would be very unusual. You can tell what your floor-to-floor height is by measuring at the stair. If you want radiant heat you'll have to remove all of the ceilings. Are you planning to get rid of air conditioning?

Have there been any additions on to the house. 7 ft ceilings were common for a short time, so cud be that the ceilings are only 7 ft in some areas. If you have an attic you should be able to see if there is any ductwork up there, or if you have a basement or a crawl space, they should be visible from there.

The master bedroom spans over the same space as the main floor living and dining room and there are floor vents all over the master bedroom which is one of the reasons why I think it’s air ducts. On the opposite side of the house, also main floor, the family room has an 8’ ceiling with a visible beam. Wouldn’t it be odd to have two different ceiling heights for the main floor? In the upstairs smaller bedrooms the lower part of the ceiling runs along the perimeter of those rooms and looks like it goes to a return vent in the master. If this is the case, do we have to scrape the popcorn before taking out the ceiling drywall or can we just cut it straight out?

About the air conditioning, the temperatures don’t get very hot in the summer and most homes in the area don’t have them.

Why would you think that you'd have to remove the popcorn before removing the drywall?

That's like stripping the paint off a piece of furniture that you are getting rid of before you take it to the dump.

Logically, I wouldn’t think it’s necessary but I’m also not a contractor so figured it’s best to ask. I haven’t been able to find a lot of information about removing popcorn ceilings other than scraping off it or covering it up with new drywall - that’s been odd to me considering ripping it all out seems like another viable option.

Those articles are about how to create a flat ceiling when you have a popcorn ceiling. It doesn't have anything to do with getting rid of the ceiling to see what's behind it.

It's just like if you googled how to remove wallpaper, you aren't going to find any articles that tell you to just rip the drywall off the wall.

Yes, I’ve researched both but didn’t find anything that addressed handling both issues at once (popcorn and height).

Regarding the height ... as others have said, it would be really weird for there to be no reason for the ceiling to be lower. So raising the ceiling is most likely an enormous expense that'd be an immense undertaking. But if you'd like to see what's behind the drywall, then rip it down. Worst case scenario, you just put new drywall back up.

You'll have to ask an asbestos abatement contractor about total removal. Who knows, maybe they have a machine that makes short work of the popcorn, and total removal might mean it all goes the landfill as hazardous waste. Every state has its own rules and regs and some of them aren't logical to a layman (or a professional sometimes).