We are getting questions about this many MANY times a day - so let’s put this somewhere central where everyone can chime in with their solutions. I’ll try to update this main OP with additional solutions as they trickle in below.

To my knowledge, at the time of this posting, there are no resources for doing this outside of hypotheticals that have not yet been tested proper. Please correct me if I’m wrong.

Please contribute to this discussion and I’ll do my best to keep it as fresh as possible.

1. Can I take the drive out of my LCD Deck and put it in my OLED Deck without issue?
   Valve has said this works well provided you have the OG Deck on OS 3.5.x prior to the transplant. Several YTers did this, but had issue booting and other problems. They do not indicate what OS they were on at the time. “Valve said” works for me - but I would probably wait for an honest real world test with regression testing follow up; trust but verify.
2. What do I have to copy from my LCD Deck to my OLED Deck to get a “full backup”?
   I’ve seen several opinions here; none tested to my knowledge. /home/deck appears to be the magic directory that if you copy everything from there - including all files and folders, technically you should “get it all”. I’m guessing this method will either not work properly or cause issues. If/when someone documents it on video - in front of you - I’d buy it. Until then …?
3. Can I clone my LCD Deck to my OLED deck without removing the drives or even opening the case?
   Yes, this should be possible. Clonezilla should allow you to clone the internal SSD on the LCD, then you can “restore it” over top of the OLED’s drive. You’re going to need a LIVE boot drive for Clonezilla, an external hard drive at least as big as your LCD Deck’s SSD and I would expect the procedure to take many hours. Again, to my knowledge this hasn’t been tried/documented. Same rules about OS3.5 on the LCD Deck first should be applicable here.
4. I don’t care about anything but my EmuDeck setup. I can’t lose that!
   EmuDeck has your back. There is a EmuDeck specific backup system offered here. I haven’t seen anyone produce a video of it yet, so I’m cautiously optimistic. This is, however, the method I plan to use when my OLED comes, so if you want to wait around - I’ll be doing it and you know I’ll include any nonsense that happens along the way.
5. Will my LCD Deck SD card just pick up and move over to the OLED Deck?
   In theory, yes. If it is JUST Steam games, the card should auto mount and the games available in Steam. What I haven’t heard about yet is if the mount point of the card WILL be backward compatible - it may mount, but it won’t “be” where the other card was (aka run/media/mmcblkp1) it might be somewhere else - so that could break “non-vanilla stuff” that is counting on that particular mount point. If anyone can verify, that would be great. If you decided to symlink system content on the LCD Deck? Those aren’t going to get magically restored - you’re going to have to do all that again. If you moved CompatData folders over there? Non-steam games are not going going to show up without being manually readded and the content on a per CompatData folder be moved.
6. Where is <content x> stored on the Steam Deck so I can back that up separately?
   This all depends on which content you mean. Some people are very concerned about custom artwork, for example. Last I checked (someone please cross check me), the custom artwork is stored in several places on the Deck - and I believe you’ll have to get them all if you want to lift and shift.
7. How are non-steam games handled? How can I move all those over without a clone?
   This represents a challenge I’m not sure how to overcome. Non-steam games are not like Steam games which all have a Steam generated ID that is the same regardless of system. But non-steam games have random ids (and hence folders) on a per system basis. You would either have to create a new non-Steam game entry and symlink/launcher command link it to the “old” folder, or copy the contents from the OG non-steam game folder to the new non-steam game folder (I am not even sure that would work). If you have a rather large collection of non-steam stuff? Clone/Image/lift and shift SSD would be your best choice.