[linux-lvm] duplicate pv change uuid

[Xen]

I want to ask again,

How can I change the UUID of a duplicated PV?

I mirrored a partition to another disk and tried to change its UUID with 
pvchange -u.

But I think LVM had already decided to replace the reference to the PV 
(partition) on the old disk by the reference to the new disk, or 
something.

For some reason it always prefers the wrong one.

Now I booted with just one disk but I cannot change the UUID of the 
running system, but if I connect the other disk I am afraid I won't be 
able to change it also.

How does one do this, again?