Alright, here is what my interpretation about the original post. When using Ellone's ability to send ones mind into the past, and by extension the device that Ultimecia uses, one takes the body of the person in the past as their own during that time. So during the course of the game, we can have
S=L
Z=W
S=K
U=E
U=R
ect.

They are not saying that one person is the other. One person acts through the other is the only relationship. So for example when fighting against Edea in disks 1 and 2, while the actuall body is that of Edea, but the mind is always Ultimecia. So one can think of Edea as being Ultimecia during that time.