The risk of going broke is "essentially zero" but the risk of moving up, and having to move back down is real.
In your example, you are playing 50NL at $700 and 25NL when you drop down to $600.
What you are doing is playing 50NL with an effective bankroll of $100.
substituting into the risk of ruin formula, you will see that your probability of moving back to 25NL is very large.
Chances are you will spend a lot of time jumping back and forth between 25 and 50, never really moving up to the next level.
If you ever do make it to 100NL, your chances of going back to 25 are still very large.