My accountant filed my family trust as a grantor type trust. I think it should be a complex type trust. Which one should I use?

I am the trustee of an irrevocable family trust in my mom's name. Upon her death, the trust will be disbursed to myself and my sibling. Last year my accountant claimed it was a Grantor type trust and no taxes were paid. This year, I'm doing the taxes and I think I should file as a Complex type trust. The trust made $18,000 in dividends. If I file as a Complex trust, then the trust owes around $4K in taxes.  Can I switch from Grantor to Complex? Should I do Grantor again? Thanks!