No, I original wrote a long reply to your questions, and just after posting that, I found my mistake. So I did a quick edit. Then went off to correct my mistake.
Anyways, it was stupid. I forgot to add the absolute path to the devices. I was in @RDM2 subfolder, and of course the devices do not exist in that folder.
With absolute path, for the devices, I can create the new RDM vmdks in the new @RDM2 folder, and was able to use those in the VM's.
Now I still cannot remove the old RDM vmdk files from the old @RDM subdir.
[root@KGC10:/vmfs/volumes/5c5c96da-3ffa9809-d5cd-bc5ff4d4f02c/@RDM] rm *
rm: can't remove 'ST32000542AS_5XW0XR9N-rdmp.vmdk': No such file or directory
...
...
rm: can't remove 'WD50EZRX2D00MVLB1_WD2DWX41D849PR06.vmdk': No such file or directory
However, I was able to rename the @RDM subdir to a different name @Bad, to put it aside. I had similar experience with this kind of stuck files in vmfs before. I could not find anyways to remove them, other than wiping and recreating the vmfs volume.