If the disciplines are related, like branches of engineering (e.g. low-level coding and rocketry in case of John Carmack), then it's not that hard. 10-15 years could be enough to get to mastery when switching branches. It's a different case when you want to switch from let's say engineering to philosophy - then almost none of the knowledge, intuitions, ways of working transfers and it could take you 20-30 years. Still, given a 75 years lifespan, should be doable.