I agree with the waiting till you have more than 20 BIs to move up. I would say 25 -30 is more like it if you are going to be moving down at 20BI. There has to be enough play to evaluate the new level or why bother (you are just hoping to get lucky and putting too much pressure to win on your first session or two at the new limit).
Wasn't it the standard advise in the past to play 5000+ hands at the new limit? No way to do that with only 1-2 buyins buffer before moving down.
If it was me, I would probably stick to 30BIs to move up and drop back down at 20 to rebuild if necessary.
Looking at it assuming you are set on moving up at 20BIs, I think the moving back down to rebuild at 15 is reasonable (although not ideal). You don't risk going broke, but you do risk having to spend more time rebuilding, where the more conservative approach gives you some protection against having to move down at all.
