My plan- Get another upper for my ar. I was wondering what wouild happen if I was to switch the bolt out between the two, depending on which one I was using. Could anything go horrifically wrong on doing this?
I used to work in the realm of naval engineering, and was always told metal parts that wear together, need to go together. But in your case of useing one bolt between two uppers, I think you'll be alright. Just don't use an old worn as hell bolt with a new barrel.
When you use a bolt with a barrel/extension, they work into each and "break in." If you go back and forth between barrels/uppers with the same bolt, the bolt will continually be in a break in process, meaning that it will just keep wearing.
For the modest cost of a new bolt, I'd just get a new bolt and keep the bolt with each respective upper. Of course, you could just swap back and forth. You might not shoot enough for it to matter.
the safest thing to do if you got an extra $25 is to get a field gauge.
If the bolt closes on it in the new upper then, no, you can't switch the bolt between the guns.
If it closes on it it means the combination of bolt and upper is over max headspace.
If there is not significant wear you should be okay. I have heard a lot of stories from individuals in the military who swapped bolts without knowing where they came from (all of which have been used previously). I however am always cautious and check with gauges, which others have suggested. It can't hurt and it is not worth the risk.
I would be more comfortable haveing 1 carrier and two bolts, rather than using only 1 bolt for 2 rifles. The carrier just holds the bolt. So there would be no worries about headspace issues.
This isn't the same situation as when you replace your bolt with a used bolt or replace a barrel/upper and use your old bolt. We're talking about going back and forth between two different barrels using one bolt. The bolt will be in a perpetual state of break in or wear in. Like I said, whether it will ever get to be a problem will depend on how much you shoot.
There are lots of people that run multiple uppers with one lower. They just switch out the uppers to suit their desired usage. When other armorers and I build an upper only, we have made it a habit to always equip each upper with its own BCG. The carrier and bolt will develop a wear pattern, as has been pointed out already. When you consider that a BCG is only $130-150, it's a sound investment. A separate bolt runs between $50-65. Why not spend another $80 and get the whole kit?
I personally recommend to people that they do not run multiple barrels on a single upper receiver, and do not run multiple bolts or BCGs on a single receiver. Instead, run individual uppers with their own separate BCGs and charging handles.