industrial building allowance

The current IBA is 4% over a 25 year period. After the 25 years have elapsed the industrial building can be sold without a clawback of the IBA. What would be the situation with a property constructed in the early 1980's when there was an intial allowance of 50% and a yearly WDA of 4%, thus effectively claiming all the allowances in half the time. Would the owner still have to wait 25 years before disposing of the property or can it be sold after the IBA has all been claimed?
Bam