I just read an article about that. It is supposed to be easier for people with copd to breath at sea level. The higher the altitude, the thinner the air, making it harder to breath and get your sats up.

my DR told me to exhale twice as long as i inhale but not to force the exhale to much

Hi, i have been flying and skydiving all my adult life until my copd stopped me. (i really miss it bad) Hypoxia is the effects of altitude on your body. When we where jumping from 14000ft we were only allowed 20 minutes above 10,000ft otherwise the affects of less oxygen would impair one's awareness. If we went any higher or spent longer up there we had to use supplemental oxygen.
I am surprised you can handle living at 6000ft. i notice it driving over the 3000ft hills west of us.
air pressure on a normal day at sea level is around 100kpa and at 6000ft it drops to 45kpa, so you really have to suck that thin air in to get any where. do you have headaches? tips of your fingers and lips blue? I'd move down to sea level. do a bit of googleing on the effects of altitude.

i once was searching youtube for breathing exercises and came across a video of a rehab that had there patients play the harmonica. so my wife got me a decent one for Christmas and a book on beginner lessons and yes it helps i get it out once in a while and fool with it and its kinda fun.

my DR told me to exhale twice as long as i inhale but not to force the exhale to much. so that would be 6 in 12 out or 4 in 8 out depending on your ability to inhale