Things you must do, do early.
Changed the type of a field today that was used throughout the codebase. Once you have a general idea of the scope of the work for this sort of task, it is worth trying to complete it, test it, and merge as quickly as safely possible. If you take too long, then some new piece of code will be come dependent on the type, and you'll have to update to fix that case too.
If you can first get everything to use the change in a structured way, e.g. via type safety, then that makes all aspects easier.