The company is allowed if they want, to transfer an amount from the revaluation surplus to the retained earnings equal to the difference between the depreciation on the revalued amount and what the depreciation would have been on the original cost. (This does not change the total of the reserves, but does change the amount that is distributable as dividend).
This is all explained in my free lectures. The lectures are a complete free course for Paper FA and cover everything needed to be able to pass the exam well.