A peaceful, East Germany-style transition would be preferable, but I also think it's possible that an invasion to free the current generation of North Koreans could do more good than letting them die under another Kim's boot. Millions live there today in deprivation and fear, not to mention the hundreds of thousands of North Koreans reportedly living (and dying, via torture and brutal execution) in Nazi-style concentration camps. Needless to say, a successful reunification of the Korean peninsula would benefit future generations of North Koreans as well.
If you're curious, here are the books which formed my impression on the subject:
"Without You, There Is No Us," https://www.amazon.com/dp/B00JTC9QBO/ref=dp-kindle-redirect?...
"The Cleanest Race: How North Koreans See Themselves and Why it Matters," https://www.amazon.com/dp/B004EWETZW/ref=dp-kindle-redirect?...
"Dear Leader: My Escape from North Korea," https://www.amazon.com/dp/B00IBYRQ18/ref=dp-kindle-redirect?...