Bankroll (again)

The risk of ruin formula is understandably flawed, since it assumes you play at a given level until you go broke.
Since we have so many levels available to us as poker players, this is obviously incorrect.

1: Assuming
a)I am playing 10/20 HE now
b)I will move down to 5/10 when my bankroll reaches X
where X= -(sigma^2/2u)ln(r) or 300BBs
how would I derive a bankroll equation for R
where R is the risk of moving down in stakes?

2: How can we apply the Kelly Criterion to the stakes we are playing in such a way as to maximize growth of our bankroll, assuming we will move up or down in stakes when our bankroll reaches X for the next higher or lower limit?