If I'm calculating a consistent rate, it's easy to model that as exponential decay. For example, if I lose 10% of customers each year, starting with A customers.
$$ y = A 0.9^x $$
However, what if that churn is less with each year. Rather than $ A * 0.9 * 0.9 * 0.9 ... $ I need $ A * 0.9 * 0.88 * 0.86 ... $ or something along those lines.
Is there a standard way to model something like that?