[T-Mobile Samsung Galaxy S6] stangdriver44:
You got it bud!!

Not sure of the other ways or if mine is easier? But it is easy and went very quickly!!

With help from you guys confirming the downgrade was possible and my desire to want to be able to build MM, but come back to my comfortable LP modded rom. I started looking...

the idea for this method came from [MENTION=1042140]tdunham[/MENTION]

After the confirmation it was possible and the idea from td I just had to gather all the necessary files, organize them and finally create ODIN flashable TAR files.

Basically the instructions are

1) download the correct files for your device from the from here

2) makes a backup or have LP rom ready to flash

3) use the downloaded files for your device and flash in ODIN

4) reboot to recovery and restore or flash your LP choice.

that's the quick answer, but that's it!! Detailed instructions below

Since my goal was to go from LP to MM and come back to LP.

These initial instructions will be for those following my same path. (already running MM, shouldn't be any different, read below)

1) using your current LP compatible version of TWRP, make a full Nandroid backup (for easy restore when I come back)

1a) Already running MM? you may need to find Nandroid and the TWRP version you used to back it up with or make sure you have a LP rom to flash immediately after this process. you will have no OS to boot into otherwise!!

2) download the correct version tar file from this folder. I made 920T for flat, 925T for Edge, and DOJC for 5.1.1, EPD1 for 6.0.1. I also linked the LP version or TWRP I used along with the MM TWRP version needed for MM recovery

3) this assumes you already have the Full ODIN tar file downloaded with ODIN in place and you know how update using the tar file via ODIN!

I will not go into that hear and this is not the place to ask how it's done!! So proceed with the FULL odin update to MM and do what you want. Root, run stock, whatever. GO TO step 4 to go back to LP

4) UNCHECK the auto reboot option. using the appropriate files for your device you downloaded from the above folder, flash it via ODIN, check the "BL" section. when it's done, unplug your phone and go to the next step!!

this step is about timing, if you don't get it the first time go again, you'll get better, trust me!!

5) hold power+home+vol down, keep holding until phone turns black screen to reboot, now quickly use the "boot to recovery" button combo (hold power+home+vol up) and hold until you see msg saying recovery in booting

6) you are in TWRP and ready to restore LP nandroid or flash LP rom! (just mentioning this, when if flashed back to LP, my MM TWRP version open up and I happened to have a zip flashable LP TWRP version so I could restore. In the past my attempts to restore a backup made from a different version of TWRP failed! So I'm pretty sure if you made your LP Nandroid with LP TWRP, you would probably need the same LP TWRP to restore. MM twrp wouldn't work for restoring LP nandroid either.) I mention that to make sure you have different versions of twrp to use for your situation.

UPDATE: Just got back on MM and all it took was flashing the Odin file I made along with MM TWRP version. Didn't need to full Odin anything. Simple as that.

My options all worked first attempt and coming back to LP fom MM including restoring and booting time took about 2-3 minutes!

I only tried this once and all went fine, but I will be going again shortly and change these instructions if needed