That doesn't really make sense. If you're moving to a new technology, even if you clean up/refresh/ fix known issues along the way, you are likely to accumulate similar such issues in the new technology as well, which would lead to a decrease in productivity. But that has never really been observed.

While I agree that the cleanup can inevitably lead to productivity gains, I believe they are overshadowed by the productivity gains from the actual change that is being made.