20-in lock ring wheel replacement

I heard a rumor that I can replace my 20- in Lock ring Dayton wheels with 22 inch tubeless dayton style rims. Anybody know if this is possible?

A 22.5" demountable rim (sometimes called "Dayton") will fit a spoke hub that took 20" lockring rims.
a 22" rim is anothertube type (lock ring style) that fits a bigger size spoke.
The next question is what you are working on. A K-5 for example, came with 7.00x 20 tires, on a 5.5 " width rim. Finding a narrow enough 22.5" rim might be hard. 8.25" is the std width in a 22.5 and that would be way too wide for the hub esp. in duels.
you would need something like a 6 or 6.5" width in a 22.5" (tubeless takes a sightly wider rim then the tubetype it replaces) and these are not common, in fact I've never seen a 22.5 that narrow.

I have said it before, but I love this forum. This is exactly the kind of information I needed in order to decide what to do. I have the capability of narrowing the wheels if I can find tires that can function with a narrower wheel. I have good tires but do not look forward to working with the lock ring/split rims when the time comes. I went to the funeral of a friend years ago that was killed with one of these wheels. And he knew what he was doing and had the proper equipment. He just slipped up.

By the way, it is my 1949 KB-5 with 750-20 duals

While I'm sorry to hear of your friends death, Lock rings are safe (the semi drop center wheel is not) and are still made new today. I run some of these OTR on my truck and change all my own tires. The key is knowing what to look for and discarding any damaged rims or parts.
As tubeless requires a "trough" in the center to be able to mount the tire over the rim lip, there is a limit to narrowing an existing rim, and there is the difficulty in finding narrow tires.
When going from tube to tubeless and keeping the same working diameter, you add one tire size (from 7.00 to 8.25) and 2.5" to the rim diameter (to allow for the trough). So to replace a 7.00x 20 you would need a 8.25x 22.5 which is not a common size and may be hard to impossable to find.
There is no reason to fear lock-ring wheels and every reason to respect a truck tire. Large tires contain a great deal of stored energy and it a sudden release of that energy can kill you.
Get a good education on any tire/rim combo that you are going to work on. Keep a healthy respect for the amount of energy you are working with. This will keep you safe.
Many truck componants that have stored energy Like wheels, spring brake cans etc, are now being made idiot proof. You can no longer buy rebiuldable spring brake cans, they are all sealed. Does this make them better? no! Does it make the stored energy go away? NO! Better to understand the dangers, and how to work with them safely.

I am comfortable with the plain lock ring wheels, but these worry me. I am not even sure of the terminology here. The wheel/rim also has a split at the valve stem opening. What are these wheels called?

I have the same wheels on my K-7. There is some different terms used. I call those true split rims, as the rim is split all the way across. Other view the Semi drop center rims of the 50's (Firestone Rh 5 deg rim) as a "Split rim".
The ones you have are a little tricky but not impossable to work with.
Make sure the bead of the tire is over the lip of the ring, and the rim joint is properly aligned.
Mount the DEFLATED rim on the hub and secure, ring in and inflate to 15 psi, and re-inspect. With the rim mounted on the hub, the rim is held expanded and can not collaps. With only 10-15 psi in the tube there isn't enough stored energy to do much. Make sure the tire/rim is still assembled correctly and the bead of the tire is over the lip of the ring. Once you are sure it is assembled correctly then inflate to the proper pressure.
This is far safer then rolling into a cage, clipping on a chuck and walking away, when the only indication you might get is when the assembley comes apart.
There are some 6" lock ring wheels out there (the more conventional lock-ring), I haven't seen narrower. Finding the 6" may be hard as well.
These rims are a bit more complicated to make sure they are assembled correctly, but go slow and be sure and safe. Best if you can have someone there for the 1st one you do, that has worked with them before.

Are you saying that the wheel should not be taken off the hub while it is still inflated? I understand why having to bolted to the hub before inflating would keep the wheel from collapsing. Thank you for your help.

No I have taken them off inflated. But inspect first to make sure the ends of the rim are butted up properly before removeing. It all comes down to inspection and respect for the stored energy.

When the tire is deflated, off the hub, and the lock ring removed, does the rim collapse in on itself ( for lack of the proper term) or do you have to fold the one edge of the rim over the other edge?