Originally Posted by
eightblack
Ung1 is right. As usual!
Without sounding silly, Gold's who continue to requalify don't see the rolling tier mile issue. It's only when you have been Gold and you fail to qualify in the window, that you will then see the rolling effect of watching the oldest tier miles drop off within the 12-13 month period.
If that makes sense. Or did I just make things worse?
So the consensus (based on ung1 and khalid26's responses, and as opposed to malct), is that previously earned tier miles do not disappear or drop to zero. Rather, the tier period simply continues to roll forward after you drop to Silver, and you could technically immediately re-qualify by flying enough to reach 50,000 in the new rolling period, which would include previously earned tier miles.
This is consistent with my brother's experiences. His review for Silver was Dec 2010, but he did not earn enough miles to requalify. He did however fly a bit in January 2011 to re-earn Silver, based on a new Feb 2010-Jan 2011 rolling period, and immediately re-earned Silver.