Consistency is best, with a slow gradual, measured movement towards 'better' where possible. When and where the opportunity strikes.
If you see a massive 50 line if/else/if/else block that can be replaced with a couple calls to std::minmax, in code that you are working on, why not replace it?
But don't go trying to rewrite everything at once. Little improvements here or there whenever you touch the code. Look for the 'easy wins' which are obvious based on more modern approaches. Don't re-write already well-written code into a new form if it doesn't benefit anything.
If you see a massive 50 line if/else/if/else block that can be replaced with a couple calls to std::minmax, in code that you are working on, why not replace it?
But don't go trying to rewrite everything at once. Little improvements here or there whenever you touch the code. Look for the 'easy wins' which are obvious based on more modern approaches. Don't re-write already well-written code into a new form if it doesn't benefit anything.