You'll have to start from scratch with the shims. If you were just setting the gaps, you would measure the gap, take the shim out, read off the thickness and then get the appropriate thickness of new shim. If the gap was too big, you'd fit a thicker shim, it it was too small, you'd fit a thinner shim. Best be would probably be to fit the new lifter with your thinnest existing shim and then measure the gap. You could then calculate the thickness required to go with the new lifter.