Short answer: Usually not for a fully-insured plan, which runs a 12-month contract and changes at renewal. Mid-year changes are generally limited to qualifying events, though switching carriers mid-year is possible in some cases (and restarts the plan year).
Fully-insured group plans are 12-month contracts, so the normal time to change plan designs is at renewal; mid-year, employers are generally limited to adding/removing employees for qualifying events. An employer can switch carriers mid-year in certain situations, but doing so restarts the plan year and resets deductibles and out-of-pocket accumulators, so it’s usually discouraged unless there’s a compelling reason. Self-funded plans have somewhat more flexibility.