If you tell people they are going to earn 20% APR by holding HBD, then you change it because of something completely unrelated with HIVE, then you lose the trust of the investors who expected a 20% return.

Leave the interest rate alone, it should be a set rate unless there is some financial problem with maintaining 20% interest then it should remain as is. Otherwise the HBD interest rate was a scam, offered 20% to get people in then dropped the interest rate once the money was in. That's false advertising, and scamming people.