s73 of the Town and Country Planning Act 1990 allows for applications to be made for the variation or removal of a condition on an existing planning permission. Different forms from those used for ordinary applications are required for s73 applications. When determining such an application the planning authority can grant the application, i.e. remove or vary the condition as requested, refuse the application or impose different conditions on the original permission.
National Planning Practice Guidance contains further information on discharging and modifying planning permissions once permission has been granted.