I was informed that if you had a figure with multiple dials, such as the astral dr strange, you could choose which dial to use before each match. Is this true? It seems like the rules of force construction would make it so you must pick one dial to use throughout the entire tournament. Please Advise.

Correct, one dial chosen and listed on your force build before the start of the tournament.
You cannot change between rounds.

where does it say that at in the rules?

I imagine it's in the Comprehensive Tournament Rules, not the rulebook.

It's something you just gotta know. I mean, it might be in SOME document somewhere, but I'm not sure which and where.

And with the Belt allowing the kooky hot-swapping at the start of each match, it's yet another reason I'm not sure why they enforced this "pick one and stick to it" rule. Horrorclix allowed you to switch it up between rounds on Crossroads figures, and it was fine. There's nothing I see that would be tremendously game-breaking.

where do you find that at?

Hot swapping?

I can't find it in the Comprehensive Tourney Rules. In fact, I can't find it laid out specifically anywhere BUT you really shouldn't need to as each portion of the figure have a DIFFERENT Set and Collector's Number. 13A is no more the same as 13 or 13B then 13 is the same as 31. Look at the Prime Figures. Does your friend think you can use Selina Kyle OR Catwoman each game? Look at the Set and Collector numbers, it's the same thing. It's also similar to say Black Adam or Phoenix. You are using a certain one (Rookie, Experienced or Veteran) and MUST use that each game. You can't go from 300pt Black Adam to 100pt Black Adam whenever you choose.

Does that help? I'll see if I can find a past thread ruling on it.

You pay to activate the slot; not for the item attached to it. Because of that you can pick what items to use at the start of each match.

I think the point is that when you submit your team list for an event, you would have to signify if you are bringing figure 021a or 021b. After listing it as such, you are stuck with using that figure for the rest of the event, just as with any other figure on your list.

Found where it was listed:
From the Final Word-
Characters with different dials
Some characters might possess either alternate dials on the same "disc" (GG #014 Astral Dr. Strange has both an A and a B side to his dial). Other characters might actually have multiple dials, separate "discs", on the same base (many Colossal characters, like Galactus or Spectre).

While each type has there own special ways to be played, there are a couple common characteristics:

You can't flip flop between dials during a tournament - For example, if I choose to use Astral Dr. Strange's A dial when I build my team, I will be stuck using the A dial for the entire tournament. Even if I think B might be more effective versus an opponent that just sat down across from me, I chose A when I built my team so now that is what I must use.