Can I switch corporate income tax filing methods (audit / expanded review) every year?

Yes. Each year when filing corporate income tax, you can decide whether audit or expanded review is better for that year and choose accordingly; you are not bound by the previous year.

For our bookkeeping clients, we calculate both methods every year and give a recommendation. You can also ask your account manager for an estimate before year end; if you are near the line between no tax (taxable income of NT$120,000 or less) and paying tax, you can plan ahead.