This excerpt taken from the BP 20-F filed Jun 30, 2005.
Corporate Transactions and other events
6.1 Save as provided in rules 6.2 and 6.3, if:
then the following provisions shall apply (unless the Committee specifies otherwise in the Grant Letter at the time of grant):
6.2 For the avoidance of doubt, Awards shall not without the consent of the Committee be exercisable under rule 6.1 in the event of a scheme of arrangement (referred to in rule 6.1(c)) if the purpose and effect of the scheme of arrangement is to create a new holding company for the Company, such company having substantially the same shareholders and proportionate shareholdings as those of the Company immediately prior to the scheme of arrangement. In that event, the Committee shall endeavour to procure that an exchange of Awards is effected under rule 6.3.
6.3 If any company (the Acquiring Company) obtains Control of the Company as a result of an event referred to in rule 6.1, each Participant may, at any time within one month of the change of Control, with the agreement of the Acquiring Company, release any Award (or Element thereof) which has not lapsed (the Old Right) in consideration of the grant to him of a new award, which in the opinion of the Committee and the Acquiring Company is equivalent to the Old Right but relates to shares in a different company (whether the Acquiring Company itself or another company its group).