this should be easy - phrase replace

[testbot]

i can't seem to get this to work in a plugin:

hook: register_activate_process

$my_msg = "something";

$vbphrase['activate_wrongusergroup'] = $my_msg;

or

$vbphrase['activate_wrongusergroup'] = $construct_phrase($my_msg);

i want to replace that phrase that already exists with something else based on a condition.