> Or is the DNA roulette wheel already spinning, like it is for people who have been heavy smokers in the past?

Just so this information is out there, particularly for prospective quitters, this is actually a bit nuanced:

https://acsjournals.onlinelibrary.wiley.com/doi/full/10.1002...

As you can see, after 15-20 years of smoking cessation, excess risk for various causes of mortality drops substantially.

You are correct that the risks never truly return to baseline, but for lung cancer, for example, the excess risk drops to between 5 and 13%.