Expertise and good verification automation tools: TLA+, Coq/Agda/Lean, specification synthesis, property-based testing, good old unit + functional testing, and elbow grease.
It won't be dissimilar to being a consultant now. I've spent years consulting where my job was to tame legacy code (that is, code which is not under test and nobody knows how it works). The value in roles like this is that you work at a level higher than the code in order to get the team able to understand the system and be able to start extending it again.
Thank you for spending the time to write this comment. As a solution architect, I don't write production-level code. I did not know of anything beyond unit / functional testing in verification automation.
It won't be dissimilar to being a consultant now. I've spent years consulting where my job was to tame legacy code (that is, code which is not under test and nobody knows how it works). The value in roles like this is that you work at a level higher than the code in order to get the team able to understand the system and be able to start extending it again.