Hasn't happened yet, so I'll know it if and when I get there...
Someone whom I call a partner (in my life, that's been only two persons - R., currently and for the last five years; and one other person for a few months during the year before I met R.) would already know about people whom I consider other close friends; so it would just be one additional step to take when I and another friend choose to call each other partners, too... I guess I'd probably mention it to R. more or less immediately if and when that label change happens.
For me, someone who isn't a close friend already won't be eligible as a potential partner, anyway... and to me, the difference between "close friend" and "partner" is mostly a question of labels to start with. *shrug*