How safe is it for someone who had been smoking for 27 yrs to move to a high altitude city of 5000 or 6000 ft elevation ?

High altitude: it depends on how your oxygen saturation levels are at sea level. There is approximately 1/3 less oxygen at 5000-6000 feet but if you do not have significant heart or lung disease and your sea level saturations are ok then it might not be an issue.

Mgt: If you have developed lung/heart disease eg COPD, emphysema or asthma, I would not suggest it. If you are still smoking, then there are multiple ways to help you quit smoking.