According to the code (from what I understand), the interest is calculated from the time-weighted amount of SteemDollar: (time_1 * amount_1 + time_2 * amount_2 ...)
That means if you change your balance, the interest will properly ajust.
If you can read code, here is the corresponding block
Thank you much for the code! You are right, luckily I understand the code now. What does this tell me? Invest in Steem and SBD