Extended time for reserved matters

I have seen quite a few examples over recent months of extended time for reserved matters - very sensible given the economic climate. But recently i have seen conditions which seek to impose an increased (but unspecified) obligation in the event policy changesafter 3 years. For example, 10% renewable energy may be ok now, but in say 5 years the policy may impose a much higher level. These "variable" conditions seek to require that higher level. It seems doubtful to me that this is either precise or reasonable but i'd welcome thoughts and any other examples.