Is it okay to switch fuel octane level from high to low?

When my parents gave me their car when I got my license, they told me to use Unleaded Plus (89) fuel every time I pump gas. I've just read the manual after finding it hiding behind a shelf and saw that my car can use Regular Unleaded (87). Will it harm my car that after 10 years of use (2001 Toyota Corolla) on Unleaded Plus, I downgrade the octane level?