Can my employer change my comp plan 7 months into the year?

r/

I am a sales rep for a medical device company. I’ve been with the company for 6 years. Every year, around March, we receive our compensation plan for the year. It’s broken down into a salary bucket and commission bucket. From Jan until new plans are received, the company holds our commission and then pays us out for those three months of sales all at once.
We just received our plans for review (7 months into the year) last week due to new leadership and rumors of a reorg. The new plan wants to slash our commission pay from 3% to 1%.
Is it legal to now make a third of the commission on all of my deals since January?
I don’t want to sign the new plan until i know if this is legal. TIA!
Location: CA based company.