Moving a branch tag to another revision fails

[jmsxp]

I searched the archives, but couldn't find the solution to my problem.
Our company has only been using CVS for about 2 years, so this may be a
newbie type question.

Anyway, one of our developers created a branch on the wrong version of
a source piece. I have been trying to move this branch tag to the
correct revision without much luck.

cvs rtag -r 1.3 -F RELV80a module.c

cvs rtag: module.c: Not moving branch tag `RELV80a' from 1.2 to 1.3.

Can anyone please provide me some insight on how to move the branch tag
to the correct revision?

I read something about a -B option, but CVS doesn't recognize that and
always returns an error.