Can a policy owner of a keyman life policy change irrevocable beneficiaries where the beneficiaries are split between the insureds spouse and the owner-employer?

Yes, the policy OWNER has the right to make changes on the policy, including changes of beneficiaries, or % of split between different beneficiaries.

Keyman life policies are usually owned by the key person's employer. The employer in this case can decide what % of the benefit the business will receive and if they want to split the benefit for other purposes (key person's spouse, trust, charity, etc).