Sephardim permit kashering an oven from meat to dairy or vice versa as a matter of course, but Ashkenazim customarily do not permit it, in case one forgets what the current status of the oven is—meat or dairy. Because an oven is expensive and there may not even be room for another one, in the event of an emergency there can be leniency. Rabbi Zalman Shimon Dvorkin, a Rabbi in 770, permitted kashering the oven for Shavuot.