That kind of clause would be void in many places around the world.

For example, the German Civil Code states:

    Section 308 - Prohibited clauses with the possibility of valuation
    In standard business terms, the following in particular are ineffective:
    [...]
    4.  (Reservation of the right to modify) the agreement of a right of the user [TL note: this means beneficiary of the terms, eg. party or other subject of the contract] to modify the performance promised or deviate from it, unless the agreement of the modification or deviation reasonably can be expected of the other party to the contract when the interests of the user are taken into account;