Cold Hard Truth About PCM Interchangeability!?!

[Blockade Runner]

Seeking to educate myself on this subject, and encountering conflicting information from GM, as well as veteran tuners. Some say you must match the "Year, Engine, and both the SERV. & HDW. Numbers". Some say the "Year & Model" (Sierra, Silverado, Etc.). Others will tell you "Any GM Truck PCM 2003-2007" is interchangeable.
Looking at acquiring a replacement/backup PCM, I found even more confusing information.
My 2003 factory PCM is "12582605/12580786" (Serv./Hdw.).
Flagship One lists this PCM as compatible with "4.8L, 5.3L, 6.0L, 1500, 1500HD, 2500, 2500HD, 3500 and Silverado SS", and that is just looking at year 2003!
On the same site if I enter my specific truck specs, it lists four different PCM's that will work in my truck.
GM performance will sell me a new 6.0L 450HP and guarantee it will work with a flash of my factory PCM, although my factory engine is 4.8L. However, I read a statement from GM on another site that says if I use a different # PCM, it will result in a "no start" situation regardless of flash.
Now I am learning, and learning the hard way that flashing PCM's is a risky endeavor. But I would like to hear from the experienced tuners from the pros that do this for a living, to the racer/restomod tweakers. My question is only concerning GM PCM's, but at long last can somebody please tell me:

What is the cold hard truth about PCM interchangeability?

[5FDP]

You could buy a 2003-2007 computer and have it flashed to your VIN number and should be able to make it work. Like say you went to a junkyard and found one, had it programmed with a GM tool and then hooked it up to your truck.

Otherwise I think you could find a 2003-2004 computer and have it work because the throttle body/TAC module and APP are the same in those years. I know for sure that 2006-2007's use different TAC modules for the drive by wire system.

It wouldn't have to be a 4.8/4L60e computer either, it could be a 5.3/6.0 computer too because we can swap segments within the tune file to make it a different size engine or flip flop between 4L60e and 4L80e.

[Blockade Runner]

5F good to hear from you.
I want to know if a PCM that will work with both cable or wire throttle body can be used in lieu of my PCM, which will only work with DBW throttle/pedal? Even such a PCM from the same year?

I also want to ask you just how cold it got the last 48 hours or so up there!?!??

[5FDP]

For a computer to work with a cable throttle body it would need to have the IAC driver on the circuit board, so you'd have to open up the computer to see if it did.

Many early computers have it and same with 2003-2005ish Express/Savanna van computers because the vans used cable throttle body up to around 2005.

It's cold, 10 degrees with 20-30mph winds and the low's will be around -10 to -15 or lower for the next week.

[2xLS1]

03 red/green PCMs can be fickle sometimes I've has issues putting 03 files in other red/green PCMs. Salvage yard interchange programs say 05 boxes should interchange with 03s. There is no one definitive answer for your original question. Many different platforms can and will use the same service number boxes with different calibrations in them.

[Blockade Runner]

I know exactly which PCM will work with a DBC. In particular the 12576106/12570558 PCM has the IAC chip down in the bottom corner. It is from the same year (2003). Mine has no chip at all in that socket.
So regardless if the 12576106/12570558 PCM came in a K-MPV or Express van, or a K truck from the same year; will it work in the 1/2T 2WD truck?

It's a long shot that I will use a DBC TB in the next engine, just trying to understand the interchangeability option. The engine does not come with a manifold and has LS3 heads with rectangular ports, so I'll have to get a new manifold/TB anyway.

I used to fly to Lacrosse Wisconsin to buy guitars. Every third business was a bar, and there were teens eating ice cream outside. I could not talk, as my face would not work in that cold. The Mississippi river would always be a block of ice with 4 bald eagles around the only hole in that ice. Everybody has a bloody mary with breakfast! I discovered cheese curds. My parts don't work that far north. Thank you for the wisdom 5F. I appreciate you.

[Blockade Runner]

2X thank you. I know I am 0 for 5 in trying to flash my '03 PCM. A bench harness will be here Sunday, so I am looking forward to yet another ass-whipping. I am learning all I can about 03-07 PCMs, and I value your experience and advice.
Give me all the wisdom you can spare friend.