I've got an old version of the board (1.6) and a new one installed on the same site but need to transfer over users.

i can't just change the ref to the user table in the options file of the new board - i tried that, it didn't work.

any ideas of how i can do this?

You definitely need to update your 1.6 to the recent version first. This is the same kind of question we have recently discussed and worked on.

After that, you will be able to achieve that both copies of the software use the same users table, specifying it under $Tu setting of setup_options.php.