Most likely, you don't. Do what you feel is best for you. This is why OWL in Atkins exists. Ketosis helps with initial hunger/satiety issues and helps get you on track, but not everyone needs or must be in ketosis to lose weight. Some studies even show that lower ketosis is better for losses.
If you like being in ketosis and/or want some of the stated benefits (this depends on you and what philosophy/theory you believe), you could get a meter and eat to it until you figure out where your sweet spot is for macros. Have you read books by Phinney and Volek? I found their book Art and Science of Low Carbohydrate Living to be helpful when getting started.

IMO, you could regain, but then again you might not regain if you're careful to add good carbs slowly back into your diet.

Sorry for such a vague answer, but at the end of the day, I think it's all very individual.