Are active duty and national guard units interchangeable? Troops?

I believe the Reserve and National Guard can replace the regular Army.

They are not

No reg army is a lot hard than ng

Only units with the same composition and the same (or extremely similar) training might be considered interchangeable. Soldiers with the same MOS might be interchangeable, but there is always a learning curve when changing units as the way if doing business from unit to unit is different. Yet, there are some specialties which are not represented in the NG, such as Civil Affairs. 95% of CA forces are USAR and the other 5% are active duty, but the NG has no CA forces at all.

I was an AC/RC advisor from 1999 to 2004. IMHO one cannot compare the ARNG/USAR force to the active force and here is why.

In the Reserve component they have, on average 39 days a year to train on specific critical tasks (used to call them METL tasks, not sure if that doctrine is still valid) compared to approximately 200+ days a year in the active force. Add to that many never get training guidance from their uptrace (none of my client units ever got it).

The RC does well things they practice, they do well in problem solving, they do well in critical thinking. But when it comes down to specific steps they tend to short cut (not a bad thing most of the time). Also think of this, if any of you had the opportunity to have your crews (howitzer, tank, stryker, or bradley) be the same set of Soldiers, for six, seven, eight years, working the same exact piece of equipment would you really care if they called each other by their first names? Would you care if crew commands were strictly followed? If they hit every target, every time, does it matter?

No, I do not feel that AD and NG troops are interchangeable in any fashion. AD troops are Federal troops. Reserve troops are Federal troops. NG troops report to the Governor of their state, and I do not believe are even federalized when they are mobilized in support of the GWOT. I could be wrong on that, but their pay issues, everything they do as a separate entity, promotions for example, goes through the National Guard Bureau (NGB).
Reserve soldiers only recently began to be able to assist and support in CONUS only if they were requested for such assistance, prior to that it was illegal.
While all of the US Army as a whole shares the same MOS pool, what they do and how they do it, are the difference between night and day. To switch out an AD soldier with a NG troop would be putting that NG troop in a severe learning curve that he or she might not be able to overcome. While making that same switch with a Reserve soldier, the learning curve might not be as steep, but they’d still be at a disadvantage. Plugging an individual into a unit situation might not be the most advantageous thing to do either. A simple move such as this, might entirely destroy team cohesion, from the point of view of the team and the placed individual. Those soldiers that train every single day together, from PT until COB, are an exceptionally valuable team, as a solid cohesive unit.
The same could be said about a Reserve unit or a NG unit, who’ve drilled together for years and years and years. I know this one from personal experience coming from my AD unit to a Reserve unit. I was the “new guy/girl.” No one wanted to make the effort to get to know me, they didn’t need anyone new. While on AD, when someone new comes in, they are not always welcomed with open arms, but they are instantly part of the family that unit makes up.
I will make the same argument that AD and NG units are not interchangeable either. When you train and train and train, and repeat the same motions every single day, you don’t have to question what the soldier to your right and left are doing. You know what they are doing, because they know what you are doing. It’s muscle memory, so that when something happens and you don’t know what to do, you don’t have to think about what to do. You rely on what you’ve been trained to do, it’s second nature. It’s not a Stop. Pause. Think. Act. type of situation. Action will beat reaction every day.
Training time as a NG or Reserve unit is precious time. You have to carefully decide what you’re going to train on each month and be prepared for that training to happen. When you sit around and tell deployment stories for 2 days a month, that’s fun, but doesn’t do anything to further or accomplish the mission. When you train and retrain on the same things every month, again, that doesn’t really accomplish anything except to become proficient at an exceptionally limited number of tasks. (These are reasons I switched from TPU to IRR.)
Now, you can take a motived IRR soldier and put them into an AD unit and not be far off the mark, shallow learning curve. I’m a shining example of that! : ) I’ve been on AD since 17 JUL 11. I’ve served long enough to get a Good Conduct Medal, and not the Reserve version.

I would say my experience as an active duty guy once assigned to the California Air National Guard that they were extremely professional in carrying out the mission. In fact they were developing best practices and had access to try out new equipment before it was fielded to the AD units. The one benefit to the guard/reserve is that they can devote more time to just the mission and not worry about the back shop extra duties that gets stuck on AD units.

I do not know about now. But during Desert Storm I was training and certifying
them to be deployed. We sent a few Vulcan units over ( I don't know why no air force to engage). Some of them could not even load the guns. To be fair it is a complicated process. But one every private in my unit could do. I had to retrain E-6s to do it! I also had to dress down several troops for not keeping their personal weapons pointed down range WHILE LOCKED AND LOADED! I will not even get into what a nightmare the M-60 and M-2 ranges were.
In all fairness I must add that some of them did know they were lacking. And almost all of them were thankful for the training and advice we gave them.

I know the Navy and Navy Reserve are not interchangeable. I prefer a Naval Reservist over a civilian but they were never quite up to the standards of a CTO in the field.

They are not, at least operationally. Guard is Title 32, state status in peacetime, while Reserves are federal Title 10 always. The Guard is a state agency, one or more of 54 in peacetime and it's often a cluster-F!