Having the solution be a one-liner as a requirement seems a little odd, since you can assign any sequence of keystrokes or any function or command to a keypress in Vim if you like.
That being said, this type of thing is Vi's bread and butter. Try:
:g/attributeA/ copy . | s//attributeB/g
where
:g/pattern/ command1 | command2 | ...
executes commands on each line matching pattern (see :help :global),
copy .
copies the current line (see :help :copy) matched by :g to after the address . (meaning the current line), and
s/pattern/replacement/g
then performs a substitution on the current line (see :help :substitute), i.e. the copy you just made. The g flag at the end causes the substitution to be performed for all matching patterns in the line, not just the first. Also note that I left the search pattern empty: Vim will remember the last search pattern used in the previous :global or :substitute command as a convenience.