Need help modifying a services contract paragraph

I need help modifying a paragraph from our firms service contract. We offer 3 modifications for certain services that the client is able to use prior to them approving the work. What is happening in some cases is clients are approving the work and we are moving on to the next phase of the project and then down the road they are changing their minds about the approved work and wanting to go back and use those modifications which ends up meaning we have to go back to the beginning and change all of the work that we have done that was based on the original approval which costs a lot of money to change everything. We need the proper language in the contract that states that once the work is approved the modifications are no longer able to be used. I hope this makes sense. Its kind of hard to explain. Heres the paragraph that we would like to optimize with the new language. Company will cooperate with Client in editing and reviewing each concept design sketch, photorealistic 3D concept renders, 3D product animation, sell sheet copy & design and webpage provided prior to completion. Client is allowed a maximum of three (3) revisions for each concept design sketch, photorealistic 3D concept renders, sell sheet copy & design and webpage before additional expenses are incurred. Client is allowed a maximum of one (1) revision for the 3D product animation before additional expenses are incurred.