What is the status of porcelain dishes that have not been used for over a year?
Can one change their designation from one (meat) to the other (dairy) or vice versa?
You do not say if the dishes are kosher to start with. Porcelain has a special status as it is earthenware covered by a thin glass layer. That makes it impossible to kasher.
R David Rosenfeld at aish.com writes
In general the practice is not to kasher them. However, in cases involving great loss, one may wait 12 months, kasher them in boiling water 3 times, and then use them (Igrot Moshe Y.D. I 43)
If the dishes were kosher to start with, this answer brings sources re switching from dairy to meat and vice versa.
Please ask a rav as the interdiction of meat and milk is serious and you should not make halachic decisions based on what strangers write on the Internet.