The interest rates will stay the same until the end of the fixed rate period that is chosen (for RIO products this could be anything from 2 years up to a lifetime). A general rule is that the longer the fixed rate period, the higher the interest rate. If the booster opts for a shorter fixed rate period (i.e 2 or 5 years), then the rate will revert to a standard variable payment. This would mean that it may go up or they can choose a new fixed rate.
For a breakdown of interest rates and monthly payments, check out our detailed guide here.