Complete newbie with probably a stupid question about tuning

I have just recently gotten into tuning with EFI Live.

I have been trying to read as much as I can but honestly don't understand much of what I'm reading which makes this really frustrating for me.
I'm fairly adept at the computer so that part isn't confusing me.

My first question is about modifying a tune without the flashscan v2 plugged in.

I have read enough (and understood enough of it) to connect my V2 to my vehicle and read the ECM to my computer.

My problem is when I try to edit that tune with version 7.5 I get a message that "Authorization has been set to "Demo"." It then describes how I must go to Help > Register EFILive to enter my FlashScan and Tune license details.

I know my serial number and have entered that but where do I find the registration key and issue date? Do I need to purchase a special license just to modify my tune file without the flashscan device connected to my computer?

I am embarrassed to admit I answered my own question. I found the license registration card in the box that the v2 came in. I entered that information into the software and can now modify tunes without the v2 plugged into my computer.

But now I have a new question. Can I full flash a different OS to my ECM? What I mean is could I take an OS from a different model Duramax LB7 and flash that to my ECM without bricking my ECM? Or could I flash a CAL-Flash only and change just the affected tables?

Yes you can, do a search there are some particulars that I can't remember though like going back to early OS's. Search for 01 to 02 OS update I see yours is a 02? there are a couple that are desired like a 6006 and 4441 OS which your truck likely already has.