It is within the board's discretion to change the date at the request of a party (Art. 15(2) RPBA 2020). A request of a party for a change of the date fixed for oral proceedings may be allowed if the party has put forward serious reasons which justify the fixing of a new date. According to Art. 15(2)(a) RPBA 2020 the request shall be filed in writing, reasoned and, where appropriate, supported by documentary evidence. The request shall be filed as soon as possible after the summons to oral proceedings has been notified and the serious reasons in question have arisen.
For the departments of first instance, see point 2.2 of the 2008 Notice (OJ EPO 2009, 68). The wording of the notice makes it clear that the decision of the department of first instance is of a discretionary nature. The request must be filed as soon as possible after the grounds preventing the party concerned from attending the oral proceedings have arisen, and shall be accompanied by a sufficiently substantiated written statement indicating these reasons. A change in the date of oral proceedings can only be requested in case of "serious reasons" that justify the fixing of a new date.
For as long as a request for postponement of oral proceedings has not been granted, the requesting party cannot simply assume that it will be. The party has to consider the possibility of a refusal of the request and has to prepare the case accordingly to minimise the risk of time pressure (T 1102/03, T 2526/11).
Where a request for postponement of oral proceedings is refused on the ground that the request was not sufficiently substantiated, the decision of the examining division should state in clear terms what should have been submitted or explained (T 447/13).
The decision to change the date for oral proceedings is a discretionary one (concerning the departments of first instance see T 1505/06, T 2526/11, T 447/13; concerning the boards of appeal, see Art. 15(2) RPBA 2020. It is inherent from the discretionary nature of the decision whether to change (or not) a date for oral proceedings that a request will not succeed simply because the reasons for it fall within the examples of acceptable reasons (T 699/06, T 861/12).
For the question of whether a request for postponement of oral proceedings can lead to a different apportionment of costs under Art. 104 EPC, see chapter III.R.2.2. "Acts or omissions prejudicing the timely and efficient conduct of oral proceedings".