On the library/internal workload side the impact would certainly not be something that fully lands overnight, but Google has a very centralized tech stack and special tooling for fleetwide code migrations. I have no insight to the particulars but I would guess there is a Pareto-like distribution of easy upgrades+big wins and a long-tail of marginal/thorny upgrades.

Google is big enough and invests enough in infrastructure projects that they staff projects like making their own internal concurrency primitives (side note, factors like this can improve/reduce or simplify/complexify migrations substantially): https://www.phoronix.com/news/Google-Fibers-Toward-Open