Commission Changes

Hello,
My company has given a contract to a small segment of employees over the last couple years that gave a yearly salary and commissions paid bi-weekly.   Our corporate strategy has shifted which will increase the commission work by roughly double which is very different from what these contracts wer designed for.  Our contract advises that we review commissions on a yearly basis so I’m thinking I can change the commission, but the salary I don’t think I can legally alter.  Would this be correct thinking or is changing the commission structure something that I could face a legal consequence from?